The best $400 I have spent for a noticable gain in sound quality was acoustic room treatments. Specifically, the 244 broadband absorption panels from GIK Acoustics. Other companies make similar products, I just happen to have had a very good experience with GIK.
Buying acoustic room treatments is not as much fun as buying new gear, but my experience has been that it provided the biggest differences in sound quality. I also realize that, depending on what room your system is in, there may be less flexiblity to have such treatments (although I ended up putting some paint on the 244s to make them look like art work.)
Some rooms are going to benefit more from this than others, but most normal size residential rooms can usually benefit at a minimum from the bass trapping that a broadband absorption panel provides (and for me the benefit was not just bass, but all throughout the frequency range). I was very surprised at how much the sound improved -- bass, clarity of voices, notes that I was not hearing before, etc.
I had borrowed a $1,500 REL subwoofer from a friend before getting the room treatments, and, in my case, the $400 of room treatments made a MUCH bigger difference in the sound quality than this very good subwoofer.
You can send pictures and dimensions of your room to GIK (or other companies) and get a recommendation with no cost or obligation. There are also some other forums (for example, Audio Circle) that have room acoustic sub-forums to get some input.