Sounds like a standing wave problem - alas, no thickness or amount of Auralex foam "traps" are going to ameliorate the problem...I've tried their offerings, and they simply don't work all that well (though I heartily endorse their quadratic diffusors). It seems like tube/bass traps are the best solution - while the ASC offering is certainly effective, you can roll your own for a lot less. Yes, it's messy and time consuming, but they are very effective. Take a look at Jon Risch's threads over at the asylum for construction info.
Good luck,
-Richard
Good luck,
-Richard