Hi power window,
What source are you using when you hear the feedback? I'll guess that somewhere in your system you may have vacuum tubes, which can be quite microphonic with higher sound pressure levels. To check for a microphonic tube, just remove the chassis and tap on the tubes in the amp (or preamp). You'll hear a "ping" sound if the tube is microphonic.
If you were playing vinyl on a turntable, the disc itself can become microphonic, vibrate, and pass on loud acoustical sounds to the phono cartridge and to your system. I have a collection of various weights and clamps for vinyl on a turntable (back when I played vinyl) to prevent feedback.
Regards,
Alan Lofft
