if your prototype version of what's being sold now is mono, that's fine.
I plan on building a stereo unit, for flexability, and cause I may as well make 2 things in one box as make one, but expect most of the time I'd be using it mono, dual mono, or maybe run in series...
Almost all the best synths ever made have just mono out (not that I own most of the best synths ever made... yet.) and I'm more into synths than anything else.
Actually, if you happen to run any analogue monosynths or old drum machines through a 660, with a comparison unprocessed file, that would really suit me! - but I assume I'm a minority with the kind of thing I'd want to run through it, so any kind of source would be very cool to hear.