Room treatments. While these can be madding to select, it offers the best bang for the buck. Check out Toole's 'Sound Reproduction' book before you make any changes.
Improve my system
I got the mod bug recently but don't know where to make my next upgrade. Right now i'm running a Yamaha DSP-A1 along with the BK ref 200.5 amp. My speakers are Kef Q70's for front surrounds, Energy APS5-2 for mains, Kef 200c for center, Kef Q70's for rears. Energy microstar 12.1 for subwoofer. I'm using a sony PS3 for my bluray player. I got about 2k to spend on a new toy. What should i spend it on?