At what vol level do you get rumble / flutter?

It doesnt sound like your TT is malfuctioning. Rumble is generally caused by acoustic feedback, particularly in TTs without suspension. Try isolating the TT on a wall shelf, pods/cones or some other method.

Forgot to mention it is on a wall shelf with long lag bolts into the studs. It is on its factory isolation cone feet.

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.