About optimal 2-channel playback : If you have some knowledge about ”the Tune method” , this is absolutely the first step to do. The cost is zero, and everyone except tone-deaf people can learn the method.
Here is how to do it :
After this important step is done - look at the first near reflections from the loudspeakers. You can use a mirror for this - put it on the sidewalls and figure out where you can see your loudspeakers from the listening position. if you can do this - put diffusion material on that spot. If the distance to the sidewalls are more than 4 meters from the loudspeaker,
no diffusion material are needed.
Because 2 channel sound is seriously flawed at the beginning, the sound needs some help from late reflections from the walls in the room - the optimal distance for this is about 25 ms = 8,5 meters delayed reflections from the speakers.
This is also (often) the same optimal distance for a choral or ensemble to the sidewalls when you are making good, acoustical recordings with only 2-channels and two microphones, in a church or in a concert hall.
Mutichannel sound is a whole different thing - you can read about how THX cinema salons are made . The cost is, again - zero, but there is a lot to learn and it might take a couple of months to understand what to do , when a real acustician can do this in an hour.