Tried tube dampers, Herbie's I think, typical of most dampers they suck the life and dynamics out of the music. Don't think they would work in your situation anyway. Your problem is low frequency bass feedback. Almost certainly what is happening is the speaker energizes the floor, this vibration conducts along and right up into your amp. Where the tubes are sticking out and like little tuning forks vibrating back and forth.
The way tubes work there is a glowing red hot plate that generates electrons. Another plate receives electrons. In between are plates that govern the flow of electrons across the vacuum space. That is why they are sometimes called a valve, tubes really do function as a valve that turns electron flow on and off.
Because electrons flow from one physical plate to another there can be a surge due to signal, or a surge due to vibration. If vibration causes the plates to swing together that is like a little surge of power. If they start doing this back and forth in a harmonic manner you get your bass feedback. But the thing is, they are doing this all the time anyway! It is just the bass feedback is such an awful obvious form you notice. But I can just about guarantee, put that amp on Pods you will notice such improvement in clarity dynamics and detail you will not believe! All due to vibration control. My amp is on a 150lb granite slab, and a BDR carbon fiber Shelf, and there was zero noticeable problems. Yet still when I put Pods in there the improvement was huge!
It will be even better if you also put the speakers on Podiums. But the smart move is to try the Pods first. That way you know for sure you are on the right track.
