I was just going to mention Bass Traps. Looks like somebody beat me to it though. Sounds like you are fighting a Standing Wave problem. Waves collect in corners of the room (two waves reflecting off of two walls 90 degrees from eachother will converge on eachother and cancel out in spots and amplify in other spots in the room, resulting in a warbling boomy effect). The tricorners at the ceiling and floor corners of the room are the main culprits. Im a sales guy, but not really trying to sell here, was just zooming through the net and caught this thread, thought my input may be of use to you all. 3-4" foam behind the speaker will help too, and in my opionion, sub woofers should energize air in the room, not the room structure, so I vote for isolating your subs as well. Check us out though: www.acousticalsolutions.com

--Later