Microphonic feedback?

Hi All

I am hoping you can help. I am running a tube integrated and a subwoofer amplifier to drive the second voice coil of my Gallo reference 3.1s. In this configuration when I turn up the volume of the subwoofer amplifier enough to activate the second voice coil/subwoofer I get a terrible bass feedback which quickly overwhelms the speaker and room. This does not happen when using solid state amplifiers. I think it might be tube microphonics ??. 
Any help or guidance would be most welcome.

Thanks in advance

Thanks MC, I’ll order some to see if that’s the ticket. I also wonder about tube dampers...?? Any thoughts?
What makes you think the feedback is related to amplifier tube microphonics? If you tap on the amp’s tubes, does it send an impulse thru the speakers?

Are you using a turntable for a source? If so, that’s the most likely origin of your trouble.
Hey Cleeds,

inpoint to the tubes as it doesn’t matter what source I am using, and this problem does no occur when I have my solid state gear in play.
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. 
If you tap on the amp’s tubes, does it send an impulse thru the speakers?
@birdfan  You didn't answer his question! To get feedback due to microphonics the tubes would have to be pretty bad. Just tap on them as cleeds asked- you'll find which are bad easily enough!