At what vol level do you get rumble / flutter?

There is a mixing of terms here.
Wow actually refers to longer term speed variations in the turntable. Flutter is short term speed variations. Neither has anything to do with what you're experiencing.
Rumble used to be the term used to describe noise contributed by the mechanics of the turntable - bearing/ motor noise etc.

Excessive woofer movement is often caused by low frequency feedback to the turntable (acoustically generated or footfalls etc) or warp induced low frequency signals being picked up by the cartridge. Since you have a wall shelf I suspect the latter.

Often these signals will be accentuated by a mismatched cartridge resonant frequency - ie if cartridge resonance occurs too low (below say 7Hz) this can emphasize these low frequency problems.
A heavier cartridge will lower the resonant frequency. A higher cartridge compliance (all else equal) will also reduce the resonant frequency.
Some cartridges just seem to be more susceptible to this issue, perhaps due to poor damping?
Not that it helps you, but I simply haven't had this issue at all for the last 20 odd years I've been using Graham unipivots.

It would be preferable to find the source of the issue rather than apply a bandaid like a low frequency filter. Not only does the latter add another device into the signal path but not dealing with a resonance problem may have other tracking and sonic compromises.

Twice your normal listening level? Turn the goddamn thing down. It's an inexpensive table and it can only take so much.

Tobes,
What would you suggest in situations where the tonearm/cartridge match looks perfect on paper, but the woofer pumping continues? I have a VPI Classic with a Lyra Delos and I still experience excessive woofer movement. The Classic stands on a 4-inch thick maple block, which in turn rests on Isoblocks. The rack stands on four heavy concrete squares and there are a few more in the rack to add mass. Granted, the turntable is between the speakers, and only a few feet away from one of them, but this is the limitation I have in the placement and it cannot be altered.

I'm just curious what your advice would be in situations where there is no apparent mismatch between the arm and the cart, you can't experiment with placement and you've done nearly everything possible to address resonance short of moving to a different apartment or spending $$$ on a state-of-the-art rack or replacing the analog front.

Actusreus, I don't have any personal experience with Lyra cartridges. I did notice in looking at the specs of the Delos that the compliance is quoted as "12 x 10-6cm/dyne at 100Hz", indicating that the compliance would be considerably higher at the cartridge resonant frequency - but this in itself shouldn't push the resonant frequency too low with a typical medium mass arm. It's actually difficult to get a really bad match. Even the highish compliance (for an MC) AT PTG-II posed no issues for me in the medium+ mass Phantom - though, anecdotally, any issues I've had in the past with other arms were with higher compliance cartridges.

I have read that Lyra cartridges are demanding and put a lot of energy back into arms. What you are getting may be the combination of the Delos' slightly higher compliance (= lower resonant freq), strong transmission of energy and the perhaps the VPI arm is not ideally coping with this. Have you played with the oil damping on the VPI?

Assessing the resonant behavior with a test record can help you see what you're dealing with. I've found there can be a marked difference between what the specs suggest and practical results.

I suspected the tonearm might be at least partly to blame. I did apply a little bit of the damping fluid a while ago, but will experiment again. Thanks Tobes.