At what vol level do you get rumble / flutter?

Its either acoustic feedback or tonearm/cartridge compatability. I doubt its a compatability issue because your TT/arm/cartiridge are all very common and medium mass. I stil say its acoustic feedback...

Ahh, the rumble. I told my dealer I had a rumble problem and he looked at me suspiciously. It did sound rather strange...

What I don't quite understand is why you would crank up the volume twice as high as your normal listening level and then notice the problem (unless I misinterpreted your post, but it seems to plainly state just that). I think a more important question is: do you have that same problem at the normal listening level? Even without rumble, if you crank up any system to twice the normal level, there will be a significant woofer excursion. There has to be to generate a much higher SPL.

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.