Even if you have the best audio gear, your room acoustics can ruin everything you hear.
The easiest way to tame your room acoustics... using a short, sweet room treatment mantra (most relevant for domestic sized rooms).
You can call it the Three Golden Rules, or Ethan Winer's Mighty Mantra.

Without treatment, all small rooms have lousy acoustics (especially in the low frequencies).

When you're listening through speakers in such a room, the room imprints its acoustics onto the sound you're trying to hear.
Forget about listening to music with high fidelity... you're listening to your room and all of its problems.

The solution: Make your room disappear.

You can effectively do this by making your room sound as neutral as possible, so that it does not color the true sound coming from your speakers.
Don't worry... by "neutral", I don't mean dead. I mean acoustically balanced for enjoyable, transparent listening.
This brings us to Ethan Winer's Mighty Mantra...
Here are the Three Golden Rules for treating small (e.g., domestic sized) rooms:

Add broadband bass traps to as many corners as feasible, including the wall-ceiling corners.
Add mid/high frequency absorption at the first reflection points on the sidewalls and ceiling.
To prevent flutter/slap echo, add some mid/high frequency absorption and/or diffusion to any large areas of bare parallel surfaces. E.g., parallel walls, parallel ceiling and floor.

Boom! Done. Of course, you could tune it further to truly optimize it for the sound you want (maybe you want an insanely accurate room, or a lively sounding room with diffusers on every wall)...
but these three steps will get you 80% of the way there, toward a "neutral" sounding mixing / listening room.