Did you take the sub out and look at it? Personally, I've seen burping in 2 scenarios.. clipping/bottoming out, or what sounds like burping, when a tinsel comes loose and slaps against the cone.
are you sure you're not overpoering them? The same thing happened to me. It's hard to believe you're underpowering them if it only happens when the gain is all the way up. Check and make sure your running enough power.