Is it better to have two matched subs or one

@elliot Thanks for that helpful post. One additional element of my setup with REL is that these are high-level inputs. Possible difference there, re: Left/Right?

Subs don't need to be identical.  I have two Rythmik subs and two SVS subs.  I have them set up in stereo, but I don't think it really matters.  They're all over my room... in front of me, beside me, behind me, facing into the wall, away from me...  I have relatively small monitor speakers - the KEF Reference 1.  I have no trouble identifying where the bass player is standing on most recordings because they provide all the cues I need and build a solid soundstage with great imaging.  Listening to the alternate version of Fleetwood Mac's Rumours I could picture where John McVie was standing.  Now when I play something like Deadmau5 or Bassnectar, the bass is everywhere.

As others of said. 2 are better than 1 and they DON'T have to match.

How low do your mains cycle?

They don’t have to match, but it’s better if they do. Any difference is more likely to make localization of the subwoofers more apparent.