Good sound is mostly setup, not spending

Only if your ceiling can use it. Atmos height effects from a soundbar work by bouncing sound off a flat ceiling, so up-firing drivers do very little under a sloped, beamed, or vaulted ceiling, or one much above 3.5 metres. In a normal room with a flat ceiling between 2.4 and 3 metres, a good Atmos bar adds a real sense of height. If your ceiling does not cooperate, put the money into a subwoofer or rear speakers instead, because those change the experience far more than height channels you cannot hear.

Use HDMI eARC whenever the TV and the bar both support it. Optical tops out at compressed 5.1 and cannot carry Dolby Atmos or DTS:X at all, while eARC has the bandwidth for the full lossless formats and passes volume control through the TV remote. Plug into the one HDMI port labelled eARC or ARC, not any other port, and switch HDMI-CEC on in the TV settings. If your TV only has plain ARC, you still get Dolby Digital Plus, which is how Atmos arrives from streaming apps anyway.

Three things cause most of it. The bar is tucked inside a cabinet or sitting below the TV firing into the base of the sofa, the TV is folding a multichannel mix down badly before it ever reaches the bar, or a night mode is compressing the whole track. Move the bar to the front edge of the surface, set the TV audio output to Bitstream or Passthrough, and turn off surround virtualisation while you test. Most bars also have a dialogue mode that lifts the 1 to 4 kHz range, which is usually worth three or four decibels of clarity.

Start on the same wall as the TV, roughly a quarter of the way in from a corner, and 20 to 30 cm off the wall. Corners give the most output but the least even response, which is exactly what makes bass boom on one seat and vanish on another. If one seat sounds wrong, try the subwoofer crawl: put the sub on your listening seat, play a bass-heavy track, then move around the floor on your hands and knees and listen for where the bass sounds tightest. That spot is where the sub goes.

In most rooms, yes, and it does more than height channels do. Real rears put effects behind you rather than faking them off the side walls, which virtualisation only pulls off in a narrow, symmetrical room. A matching wireless rear kit runs somewhere between 150 and 350 dollars, and you need a mains socket behind the seating, which is the part people forget until the box is open. If getting power to the back of the room is impossible, a better subwoofer is the next best spend.