Opinions on stereo subs v. a single mono sub?

Two reasons that I can think of are the point and slope of the cross over - if it is too high or two shallow some signals (in stereo) will become a mono source from a 3rd location. This will muddy up the clarity of the image in that frequency range. Also two subs don't have to work as hard as one to produce the same volume. You can use a sub with a smaller and faster driver and get the same effect, but a much cleaner one.

With LP source, deep bass is monophonic. Records are made this way because strong vertiucal groove modulation tends to make the stylus hop out of the groove. Also rumble filters often reject the differential signal (vertical) because that is what turntable rumble mostly is.

So, if you play LPs, a mono subwoofer is just fine.

Digital sources do not have any reason to blend the LF to mono, and in many cases the LF (for example: a bass drum) is plainly in one channel. A mono subwoofer looses this spatial effect.

i run my vmps large sub in stereo(crossed at 90), tried it in mono last week for the hell of it-in my system it was louder in mono(outs from my rxv1)but it covered some mids , switched back to stereo outs and it is better imho.
stereo bass is goooooood-lol