Does anyone use a RUMBLE filter?

A passive 6db/octave hi-pass filter, which could consist of a single low value/high quality capacitor in series with the amplifier input might be quite transparent. However, the cut-off frequency would have to be too high up into the audio range (guessing around 50Hz) to be effective at the very low frequencies for "woofer pumping". To implement an effective rumble filter that cuts in only at sub-audio frequencies, e.g., below 20Hz, I think you need a slope of at least 12db/octave or higher. Then, if you do that, you are looking at insertion loss, several components needed to get the steeper slope, etc. To avoid insertion loss, you might need an active filter. All of the above leads to loss of transparency and fidelity that can affect the entire audio range. So in this instance my bias is the same as Raul's. Less is more. But this is just "in principle"; I am not about to say that someone else's system cannot sound better with a well designed rumble filter vs without it.

Right now I am using a pair of Transmission Line woofers that I built several decades ago as bass support for a pair of Beveridge 2SW ESLs. (The 2SWs were designed to operate from 100Hz up.) Long ago, when I built the TL cabinets and used them subsequently, woofer pumping was always an issue because the woofer in a TL is essentially undamped by the cabinet. I am rather surprised that I see zero evidence of unwanted woofer motion, and I wonder whether the Dynavector tonearm I am using is more resistant to the problem than most.

Maybe it would be more constructive for Raul to say how to get rid of woofer pumping without a rumble filter. Since Raul (and others) have posted elsewhere that the interaction of tonearm effective mass and cartridge compliance can be disregarded with good results, would it be fair to say that mismatching those two parameters is a cause of woofer pumping and constitutes one instance in which we should pay attention to those hoary equations?

Warped LPs are an obvious secondary cause. But we don't play warped LPs very often, do we? (We are connoisseurs.)

That's a reasonable answer. I think that even when there is not easily visible woofer pumping, "full-range" speakers can suffer a bit from the introduction of distortions that originate from extreme low frequency (sub-audio) signal that moves them around while they are trying to reproduce audio frequencies. I believe this represents your position, as well. So one solution is a sub-woofer or separately powered woofer. But that introduces new problems of integration and from distortions introduced by the necessary crossover network. There's no free lunch.

Raul, Yes, that's the way I would do it too. Use only a small value very high quality cap as a high pass filter in front of the amplifier that drives the upper frequencies. Then use a very high quality electronic crossover, with a steeper slope, to derive the low pass filter that drives the woofer amplifier. That's what I am doing with the Bev 2SWs, and I am very pleased. With a little fiddling, the crossover point is not audible. This is relevant to the topic, because by introducing the separate woofer/subwoofer, one is removing any rumble distortion from the main speakers without having to resort to a "rumble filter".

And yes, the success of that set-up has me thinking about adding a subwoofer to the Sound Lab system.