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)