Why are my woofers pumping?

Dear @o_holter : According your self research looks like the rpoblem is developed by the TT and not a mistmatch between cartridge/tonearm because even at 100hz the Atlat compliance and effective mass puts the resonat frequency in a " solid " 8hz.

The cartridge is a very sensitive " microphone "/transducer and it's picking up " something that is happening somewhere in the TT.

Normally when  a some kind of resonance/vibration are exiting/coincide with the cartridge/ tonearm resonance frequency in between the pick up jumps more than make a noise.

Before you go for the filter try to " figure " what could be happens in your TT. You are the one that know it, even contact the TT manufacturer and ask for its advice.

Regards and enjoy the MUSIC NOT DISTORTIONS,
R.

Your problem is most likely a combination of ununiform belt thickness, not precise pulleys, motor rumble, not precise bearing. They all combine and create a mechanical shimmer in the turntable's suspension. 

While playing the runout groove and watching the woofers move in and out. Gently, but firmly place your hands on the plinth and try to very carefully hold it in place. The woofers should move in and out less.

Still even though it still falls within the 8hz range, I believe the ideal range would be closer to 10hz.  so if you buy a cartridge that you think has a compliance of 12 and it is really closer to between 16 and eighteen, i would think that would be audible at some frequencies which could be bothersome for some people.  its kind of deceiving as I did not even notice the whole 100 hertz thing until after I bought my cartridge.  It still sounds good but it is not what I thought I was getting.  for instance when I bought a benz cartridge and its compliance was 16, it was 16.

tzh21y - yes I think the same. Maybe even small things help, getting the resonance up. Stripping the arm of all extra mass and changing the counterweight position, like Atmasphere suggested.
Invictus, Almarg - I don’t think the player is the problem, behaves much better than my former VPI. The plinth is dead stable as it is. Belts - maybe new ones would be better, but this is not the main cause.
Somehow, the pumping has decreased a bit over the last days. Not sure which of my modifications that made the change.
The pumping is now very evident when I turn the volume up to 92-96 db from the listener position - measured when playing music. Going down to 88-90, where I usually listen, it is not so notable. I must test this more, trying to find "silent" vinyl grooves.
Atmasphere - I was not aware that loading could influence the resonance - ? I load the Atlas at 475 Ohm, but could go down to 100, have not tried yet.

Why don't you try what I posted...