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

birdfan