It isn't acoustic feedback as I had the same problem. After trying everyone's advise here, I bought a KAB rumble filter and it solved the problem. It goes on a tape loop and it does not take away any bass, just the oscillating.
Low frequency problem while playing vinyl
Has anyone experienced (and knows a cure for) a very low frequency problem while playing records. Just recently I hooked up two old turn tables, Linn LP12 (1980s) and Dual 704 (1980) in my system and the woofers of my speakers are almost jumping out of the speaker cabinets while I am listening to a record. Displacement at least 0.5 inch if not more and the keep "pumping". Needless to say that I didn't experience this issue years ago when I used to play records regularly.
amps: Krell Ev202/350MCx
speakers: Thiel CS7 + smart sub
phono amp: Valvet
amps: Krell Ev202/350MCx
speakers: Thiel CS7 + smart sub
phono amp: Valvet