Swap amps too. Some of the resonant sound that you're hearing is the parafeed circuit too, not just speaker panel resonance.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.