Woofers getting workout, but no bass

You might play with the way your counterbalance is set up. The Triplanar has a multiple weight system and you can use that to affect the effective mass of the arm and cartridge.

If the effective mass is off, the setup might be susceptible to warp and the like. I have the same arm and cartridge, with no problems and no rumble filter.

We have the large weight and one of the smaller ones (about 3/8" thick) set fairly close together to get our tracking pressure. The amps and preamp have bandwidth to 2Hz no problem, but no problem with woofer pumping either, even with some pretty warped records.

Audiotomb
I think doing a capacitor change in the phono stage to roll off the lowest freq would accomplish what the KAB would do without all the extra hardware and interconnects.

Why did manufactures get away from subsonic filters?

purity of corrupted sound

If a capacitor change fixes your rumble problem, that's great. I doubt it will as this problem seems to be inherent in analog playback. I've seen six-figure setups where the woofers were going crazy, regardless of the tonearm/cart matching or superb isolation. My setup matches perfectly on paper, but I still had woofer pumping notwithstanding until I got the KAB. Until I see a full range analog setup with no woofer pumping, I will remain skeptical about the claims it's all about tonearm/cart matching or resonance control.

Also, I don't think manufacturers "[got] away from subsonic filters." To the best of my knowledge, it has never been a regular feature in preamps. Some incorporated it in their designs, some didn't. Same holds true today. Mega-buck phono preamps, such as Boulder phono preamps, still feature it. One could argue the fact that manufacturers offered it to begin with indicates that they recognized it as a common issue that needs to be addressed.

Finally, your "purity of corrupted(?)sound" remark ignores the fact that if your woofers are fluttering, you're not getting "pure" sound whatever that means. The speakers are going crazy trying to reproduce subsonic frequencies and your amps' power is being wasted feeding these frequencies into the speakers. This, combined with possible damage to the voice coils, is enough for me to justify using a good quality subsonic filter.

...this problem seems to be inherent in analog playback...

There would be quite a few who would disagree, including myself. However this is IMO/IME a pitfall of improper setup!