Phono-pre: subsonic filter on or off ?

The reason that it's there is that not all users need it. So that begs the question. If you turn it off and your woofers start pumping, or you have acoustic feedback, you need it. If not, you don't.

The roll off at the bottom of the audio band is usually pretty subtle, though a few are actuall down at 40 cycles, you can verify this for yourself by turning it on and off and listening. On the other hand, it will most certainly have a marked effect on phase response in the audio band, and the jury is still out on that one, as well. So it all comes back to trying it both ways.

In addition to Viridian I'd turn it on when I play warped records.

They vary widely in effect in my experence. If I didn't have a problem I would leave it off,I do: if the woofers flap wildly then use it. It does have an effect on the bass but if you need it you need it. One of my phono stages has both RIAA and IEC {?] curves; the latter rolls off the bass more. See which works better on your system, off or on, and go with that.

Use it IF you need it. The only reason to use a subsonic filter is to avoid the woofers rumbling, or subsonic moving in and out. If you have enough isolation of TT from the low frequency output of the speakers, and have no problems with low frequency feedback, you can leave the subsonic filter off.
(i have never needed to use one)

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.