Phono-pre: subsonic filter on or off ?

Off if possible, but on if needed.

Whichever sounds better in your rig on particular recordings.

If you leave it off and it is needed, your amps power gets used up producing low frequency noise leaving little for the music,which can be a precarious situation in regards to sound quality at best and potential damage to speakers at worst.

If you find it to be a significant problem on a consistent basis, you can eliminate or at least reduce the problem by tweaking the phono rig configuration (cart and tonearm matching) to change the resonance frequency, but that is a more significant undertaking that requires some expertise up front.

Better physical isolation of the table from the sonic energy transmitted from the speakers via the air and floor can also help. For example, if the turntable is sitting high up on a less rigid platform, like the top of a cabinet in a room that is on an upper level of a dwelling rather than sitting on the solid concrete foundation, move it lower to the floor on a rigid, solid platform and away from the direct line of fire of the speakers.

It also depends on what frequency the cut off is omplemented. If at 70Hz, then you are definitely losing something with it on. In which case I would leave it off unless your woofers are pumping hard from rumble.

If it cuts off at 30Hz or below then it may not make much difference leaving it on all the time

Ptm,
I've never seen rumble filter adjustable. It's mostly oriented on sub-sonic frequencies of a large amplitude. Hence the alternate name is SUBSONIC filter.
SUB-SONIC means UNDER-AUDIABLE.

Actually, subsonic means 'under the speed of sound'. The proper term is Infrasonic. We don't say sub-red for infra red do we?

Also, Carver produced an infrasonic filter that automatically adjusted its cutoff properties to the low frequency content of the recording and the turntable's inherent rumble or lack of.
Salut, Bob P.