That "boom" situation sounds like it could be more of a placment/integration issue than a hardware problem. Sometimes, if the sub is crossed over at too high a frequency it can put undue emphasis on the upper bass, thereby causing that boom thingy.
Likewise, if it's in a bad location, it can add undue reinforcement to a narrow band of frequencies, thereby creating that boom thingy again.
Or, it could be a combination of both. I'd experiment with different placement scenarios first.
Oh, one last thought... if the mains go down low enough, they could be adding to the boom problem as well, particularly if they are running full range.