Audio and Video
The most important thing for an immersive viewing experience is how much of your field of view is covered by the (moving) image.
However, due to the geometry of the eye, it's better if the distance to the TV is large, so a TV with a large diagonal situated at a large distance is experienced better than a TV with a small diagonal situated at a small distance.
The resolution of the image has little relevance because you are viewing the image in its entirety, not pixels, and this is even less relevant for moving images (because the resolution of they eye is reduced in such a case).
In cinemas, the recommended distance can be as low as 0.85 times the screen's diagonal, but that type of experience is for movies. If you are overwhelmingly watching TV shows with normal or little movement, the viewing distance to the TV can be around 1...2 times the TV's diagonal. For TV shows which have more movement, for longer, like sports, it's better if the viewing distance is 2 times the TV's diagonal, else you could get motion sickness.
For a cinematic experience, the TV needs to have a diagonal of at least 200 centimeters (80 in).
If you care about image quality, buy an OLED TV because of the consistency of the colors regardless of the viewing angle.
If you want buy an 8K TV, note that the resolution difference from 4K can barely be seen from up close on a 65 inch TV, so you should either buy a significantly larger TV, or buy a 4K TV (because it's cheaper). Don't choose 8K over OLED.
Keep in mind that due to the potential of image retention, an OLED display should change the image from time to time, meaning that it's not desirable as a computer display.
For a true cinematic experience, look for a UST laser projector and an ALR screen.
UST means "Ultra Short Throw", that is, the projector can project an image with a diagonal of 300...450 cm (120...180 in) from a distance smaller than 30 cm (12 in) from the the screen.
An UST projector can project the image only starting from about 30...40 cm (12...16 in) above itself, meaning that the top of the projected image might end up on the ceiling if the projector is installed to high from the floor, or if the image is too large.
A laser projector doesn't have a lamp, which means that it doesn't get hot (during operation) and doesn't require lamp changing (changing which is expensive).
ALR means "Ambient Light Rejection". An ALR screen provides a black level which is vastly better than that of a non-ALR screen, which means a vastly better image contrast, to the extent that images projected (with a UST projector) on an ALR screen during the day look as if seen on a TV (in the same context). This means that ALR screens make projectors usable in daylight.
You can also find ALR paint, which is much cheaper than an ALR screen, but its black level is not as good.
For ALR screens, search for "Black Diamond ALR screen" and "Vividstorm ALR screen".
For ALR paint, search for "Nox screen".
The absolutely most important thing for speakers is not their "quality", but how they project the sound in your home. That's why you're buying speakers and not headphones.
This means that the distance to closest walls, the shape of the room where the speakers are place into, the material of the walls and the room's content will make a dramatic difference, whereas for headphones these factors are irrelevant.
For the audio system, you need a left and a right speaker, so look for a good 2.0 passive speaker system and an amplifier.
Don't waste your time with whatever X.Y speaker systems. Put the money where it matters: in those two speakers and the amplifier.
For the speech in movies, a center speaker may improve the immersive experience because people usually hear voice from the direction they look in (because they focus their attention that way).
Some amplifiers have a feature which automatically optimizes the sound for the space where the speakers are place into; the use / come with a microphone for this.
If you are serious about sound, hire an acoustics engineer to design the (music) listening room.
Are audio systems without a subwoofer any good?
Yes. Such systems can't use a subwoofer to hide the fact that the satellite speakers are of low quality.
An integrated subwoofer is used by manufacturers to cover the weakness of the satelite speakers in reproducing a perceptually balanced sound, by overpowering all other frequencies, especially the mid ones (present in voice), with bass.
The integrated subwoofer will make the music sound powerful in the very low frequencies, but will further reduce its clarity. Because the music lacks clarity and sounds muddled, people will turn the volume up to hear better (but this won't improve the clarity).
Cheap speakers usually have an overemphasized bass because (it's cheap to do and) people don't like to buy speakers that sound tiny / thin (like radios). However, they might have a "Bass" knob that you can use to change the amount of bass they output.
To see if your speakers use bass to hide a reduced clarity in the mid to low frequencies, deactivate the subwoofer and turn off any bass effect (from the "Bass" knob on the speakers). Now listen to voice (with no music); the average YouTube voice-only clip is more than enough for this.
If the voice sounds tiny / thin, similar to that of the average radio or smart TV, but possibly less clear than a radio / TV (because the radio / TV is designed to output clear voice), you have evidence that the speakers lack mid to low frequencies. If you add bass effect, the voice will start to sound boomy instead of getting fuller; this is because bass adds very low frequencies, not mid to low frequencies that voice needs to resemble human voice.
Why can a radio / TV be designed to output clear voice, while a speaker used for music can't? Because the radio / TV doesn't actually improve the quality of the mid to low frequencies of voice, it decreases the power for other frequencies that would normally cover the voice. If you were to do the same with a speaker used for music, the music would sound bad / underpowered.
Why then do humans have such full / deep voices? Because when humans speak, their chest resonates, and because the chest is very large compared a to speaker's driver, it powers a lot more the mid to low frequencies, giving voice its fullness.
Wireless headphones, WiFi or Bluetooth, can provide a sound clarity which is much better than that of most (standalone) speakers. On top of this, since they are on the ears, you can hear every sound from the music.
Some headphones have both WiFi and Bluetooth.
Headphones can only make you hear the thumping bass, they can't make you feel it on your body (like subwoofers can), but they also don't stress the neighbors.
The headphones whose ear pieces completely surround the ears, called "over ear", are usually the best for comfort (because they don't press your ears, unless the earcups are not deep enough) and sound quality (because they have the space to contain better technology).
For the same sound quality, wireless headphones are much more expensive than wired headphones. Also, you can't find audiophile-level wireless headphones, but for wired headphones you can go as high as you can afford.
Headphones make room acoustics irrelevant, while for speakers it's very important.
Open and closed
There are open and closed headphones. Open headphones allow the air (and sound) to circulate between the inside and outside of the headphones, through the outer side of the earcups.
Because the sound exits from the open headphones, they can disturb the people in the same room.
Closed headphones don't allow the air to circulate, and because of this their sound has a very faint echo (you will not be aware of it) which may make some types of music (like rock) more interesting.
Because the air doesn't circulate in closed headphones, the ears get hotter than in open headphones.
Headphones with noise canceling are closed because they need to be as insulated from the outside as possible.
Unless you want closed headphones because you don't want to bother people in the same room, you should get open headphones.
Soundstage is the size of the area from which sound appears to come, when listening to headphones or speakers.
A wide soundstage means that the sound feels like it's coming from a wide area around the listener, while a narrow soundstage means that the sound feels like it's coming from a narrow area (like listening through a hole).
A narrow soundstage makes sound perception less clear, so listeners feel the need to turn the volume up to hear better.
Open headphones have a wider soundstage than closed headphones. Large and deep earcups increase the soundstage.
Because the headphones with noise canceling are closed, their soundstage is limited.
See Rtings for headphone ratings of technical parameters like soundstage.
WiFi headphones come packaged with a base station. The base station has to be connected through a cable to the 3.5 mm headphone output of a TV, portable radio, computer or standalone music player. The headphones themselves connect to their base station through WiFi, to receive the sound, and this wireless connection lets you move around from room to room, while the base station and the sound playing device stay in the initial room.
Proper headphones don't have a problem if between the base station and the headphones there is one wall made from reinforced concrete, but two such walls will likely be.
There are models of headphones which allow you to connect several of them to a single base station, so that several people can listen to the same sound, at the same time. Usually, you can't buy a single base station and several headphones, you have to buy as many headphones as you need, each packed together with its base station.
The headphones can be placed on the base station to recharge the batteries. When the batteries are new and full, they can let you listen to music even for 20 hours.
Bluetooth headphones are usually made to work with a music playing device which contains a Bluetooth emitter, like a smartphone, so, unlike WiFi headphones, the base station of the Bluetooth headphones doesn't connect to the music playing device, but is used only to recharge the batteries of the headphones.
Because Bluetooth headphones are mostly made for portable devices, some models have a noise reduction (/ canceling) feature which significantly reduces the noise that comes from the outside into the headphones, so that your music listening experience is not degraded by the the surrounding noise. Be very careful with such a feature because it will also limit your ability to hear incoming cars and other potential dangers. Activating noise canceling will likely degrade the sound quality.
Bluetooth headphones are likely to have a microphone will lets you answer phonecalls, but privacy oriented people might prefer to avoid that.
For the same sound quality, Bluetooth headphones are more expensive than WiFi headphones, but have more features.
If you hear short interruptions in the sound of the headphones, increase the volume of the source (PC or smartphone). The emitter is optimized to cut power when sound volume drops to near silence, so the headphones don't receive anything during very quiet periods.
Some headphones have rechargeable batteries, usually one in each ear piece, which can be replaced when their charge lasts too little time (like just a few hours). For some headphones, in order to open the compartment of the batteries you have to rotate (in opposite directions) the outside and the inside halves of each ear piece.
When you buy wireless headphones, in order to test their sound quality, make sure to disable any effects (like noise canceling) on the emitter.
Using an expensive instead of a very cheap cable between the sound source and the base station of WiFi headphones may make a perceptible difference in sound quality.
If you're using an optical digital cable to connect the sound source and the base station of WiFi headphones, you may have to set the sound mode (in the source) to PCM.