Woofers getting workout, but no bass

I'm puzzled: I'm noticing just about every LP I've played in the past couple days has my 10" woofers moving like mad, but not in time with the music and regardless of whether there's any audible bass.

I can only conclude that I have subsonic frequencies occuring (my phono stage doesn't have a filter). What would cause this?

Relevant components:
Speakers: Wilson Sophia 2's
Turtable: TW Acustic Raven One
Phono stage: Tron Seven

Thanks in advance.