Kendrix,
> I take this to mean that room modes in the low frequencies are not causing too much trouble. <
I wouldn't be so optimistic! 
But what are the dimensions of your room?
One reason you might not have noticed this - yet - is the location of the peaks and nulls depends on the frequency, which in turn depends on the key of the song.
The problem in small rooms is not only that the response changes, but also that it's not flat anywhere. A great way to determine this is to play various low frequency sine waves one at a time, and walk around the room. That is, play 80 Hz and walk around, then play 100 Hz and walk around, and so forth. If you don't hear the level change by huge amounts I'll be very surprised! So please try that and report back. I assume you have a way to generate steady sine waves? If not, you can download a Windows version of the NTI Virtual Minirator for free HERE .
--Ethan
