Woofer pumping - what gives?

Thanks to all for the responses. Mike's description of the exponential phenomenon is spot on. I ran some comparisons tonight and the dramatic looking woofer-wubbing occurs only at rock-approved levels or slightly below; equally with the dustcover on, up or off; equally between music cuts or during them; and not at all, regardless of volume setting, with the stylus out of the groove, the platter spinning and the tone arm cued up. The stylus rides flat on the test LP's so I doubt warping is the issue, unless you consider a record groove to be a warp of the vinyl blank.
I never noticed the wubba effect with the grills in place. Though I have new admiration for their wubba hiding properties, I'm leaving them off anyway.
The whole rig's fidelity to the suggested subsonic signature even with no recorded signal present makes me wonder: is subsonic woofer wubbing inherent by nature in the LP playback process?

No. No wubbing on my rega now. That's a totally unsuspended deck. I've got to hand it to my $10 block of concrete and $5 foam slab.
Of course I could have gone to a dealer and bought audiophile concrete for $150.

subsoninc feedback is definitely your problem. Isolation is the key, and also try moving your rack out from the wall, where low frequency waves stack up.