Why are my woofers pumping?

@o_holter The studder you are seeing in the spindle is typical af asynchronous motors and not a malfunction.  Download and read the manual for the VPI HW-19 MK 4.  There is a paragraph that explains this.

If your woofer pumping returns do not use a filter.  You would just be covering up the problem instead of solving it.

in the meantime, enjoy the music.

Thank you vpi. Always something new to learn.

Some more testing. The woofer pumping problem is still there, maybe a bit reduced, but not fully solved (sorry for too general statements above - I thought it was solved). It is very dependent on the record - some LPs have more, some have less. I am investigating whether a filter could be an idea, using a laptop and a sound card to see what goes on in the low hz region (through the program Audacity).

vpi, and others - two camps in this thread, "fix the cause" or "fix the problem" - if I want to fix the cause, and change the tonearm, keeping the Atlas, although I dont have the financing to do it now - what would be a good match, with a resonance around 10 hz? That sounds good with the cart?

One more test. Changing the connectors from the Aesthetix Io phono stage to the Einstein preamp from balanced to single-ended. Result; much less woofing with single-ended. But much poorer sound too! More flat, congested, less involving. Even when I turn the volume up, to get the equivalent decibel level (which I have to do, for the single-ended connection), there is less "here-ness" to the music. I have a semi-balanced phono stage - balanced from the second amplification section through the output. The balanced outputs sound way better than the SE / RCA outputs. Conclusion: a single ended filter like the KAB probably will not work for me. Running the phono signal single ended, or through a single ended tape loop, will degrade it.
Cables used for testing: 1) balanced - JPS Superconductor 3 (very good xlr cable); 2) single ended - Kimber KCAG (very good in some applications but not here).