Do the woofers "wobble" when playing stuff that has no low bass? If so its not feedback. Im guessing Sktn77a has got it right.
Woofer pumping - what gives?
Some years ago I noticed that regardless of the speakers, cartridge or preamp in the circuit my woofers pump wildly out of control the more I advance the volume while playing an LP. They don't seem to be responding to the musical signal but something else. The sound is fine. My humble rig is a Dual 1219 with a NOS Shure M91ED cartridge. Tapes made from the same LP's do not exhibit this odd behavior. Who knows why woofers look like they're going nuts while playing an LP?