I heard this kind of resonance on some of Johan Van Zyl's speakers during a recent trip to Dallas. It sounded just like the speakers were slightly underdamped, or a panel was unbraced. I mean, it really sounds like the speakers. But I knew that the amp he was using gave a little bit of bass bloat just like that, because I had one just like it for a couple years. It sounds good, but tends to make speakers have an underdamped sound. It's a pretty good match for very small high Q single drivers, but it's a little thick for larger, more balanced speakers. So I went out to the car and brought in one of the amps I took to show and viola! Perfect! No more bass resonance. Swapping out the amp to one with more bass control gave his speakers the balance they really needed.

I like your amps, and think they sound nice in the mids, and provide deep, full bass. That's a nice change from some of the other low power SET amps that don't have any bass response. But the bass is a bit bloated, and it will be hard to tell that symptom from speaker resonance. Trust me, it isn't all the speakers.