Unsolvable Woofer Pumping (Phono only)


I'm at a loss for trying to find the source of my "woofer pumping."  It's most noteworthy when playing something that is mostly/all treble, and the woofers of my Focal Aria 906s are going nuts (inaudibly, of course).  Turntable is a Debut Carbon with Ortofon 2M Blue.

Initially I was told it's an isolation problem, so I better isolated my TT, even put it right on the concrete floor to test!  Next I thought maybe a problem with the TT itself, so tried a couple others, no change.  So I figured it must be acoustic feedback, as with the TT stopped and stylus on a record, I could produce woofer pumping by tapping on certain parts of my stand...but it is also not this! I turned off my amplifier and recorded from the pre-out to a Tascam digital recorder and played that back afterwards and the pumping STILL happened! So I tried an Schiit Mani phono stage, no change in woofer pumping...I was sure it had to be my pre-amp...

So a local audiophile came over with a couple of pre-amps and we tried those.  The only time the problem went away was when the subsonic filter that one had was engaged.  So, I've ordered some Harrison Labs "FMODs" (20Hz high pass) to see if they will help.  If they do, I may order a KAB RF1 one day...but don't want to spend that much if I don't have to.
Any other ideas on what could cause this?!

tl;dr: Woofer pumping not caused by isolation, acoustic feedback, phono/preamp or a compliance issue...what's happening?!

branden_8091
As raindance said (and I implied, too), subsonic filter is what you need. You even mentioned that the issue resolved when you applied subsonic filter. 1+1=2. Now, you can try to figure out why subsonic filter is what you need.
@agp46 I do not (yet!)...I've 'solved' the problem though, turns out it was just uneven or slightly warped LPs... :(
@glupson I know that that fixed the problem...but I wanted to find the root cause. I finally did though!  Just uneven records :(
Do you have many warped records? Could it be that you are storing them horizontally, on top of each other? Or exposing them to heat?
@noromance It seems only maybe 1/5 are bad enough to cause the problem I was experiencing.  All stored vertically in a cool dry place. But, many are used, so who knows their past? I try to stay away from records with the telltale 'ring' on the cover from being stored horizontally.