Then it sounds like you would be more interested in room treatments, as opposed to isolation and sound proofing. There are a few people here who can probably give you more details, but I'll start you off with some general reading. Do some google searches for:
- room treatments
- bass traps
- acoustic panels
(easy ones to start with, and to DIY)
- hemholtz resonators
- quadratic residue diffusers
- binary amplitude diffusers
(more complicated and specific, but can contribute a lot to fine tuned rooms)
Forgot to add that you will probably want to also look into something like Room EQ Wizard (REW) which will be able to tell you a lot about the sound in your room, and what you need to do to improve it. This stuff gets complicated, but the more detailed you get, the better your sound will be, so hang in there. You might also want to check out the Master Handbook of Acoustics for some solid theory and understanding.