Hiya T!
-> I have never played with "surround sound", but I certainly wouldn't
-> bother running subs in stereo, since at those frequencies sound is
-> essentially mono to begin with.
While I would normally completely agree with that, in my studio monitor
system, I do run stereo subs right now, but that is because my top
speakers are -3dB at 120Hz, so I have the subs set for around an 80Hz
Fc, While even at that frequency they are probably still not
directional, I still run them that way in the studio.
 The setup originally intended for a keyboard sound system for small
venue live peformances would probably be setup with both subs together
and the amp run in mono for live gigs, because of the couping of the
cabinets and getting the additional power from bridging the amplifier.
As far as a theatre sound system or home theatre, also a single sub
would be typical but at lower frequency on the crossover than the above
example. There was a configuration in the original Dolby Stereo
specification for 70mm using two sub channels, but that has long since
been abandoned for a single sub channel in most systems.
Bonnie *:>
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