Thanks for your input, buttercup. I agree; most guys I see around my neck of the woods are using ported cabs, but for my taste, that tight, punchy sound you get from a non-ported cab with sufficient power is the ticket. Even with my GK 1001RB-II, I would agree that my single 210 SBX-II would probably not cut it in a big room with a loud band, and that the bias seems to be toward 410 cabs.
But! 2 - 210 SBX-II's will move the same amount of air as a 410, and are considerably more portable. That setup will also give you a 4 ohm load to let the amp do all it's capable of. I bought mine with the intention of adding a 115 SBX-II (or another 210 SBX-II), but they are out of production and very hard to find used. I'm not in a hurry; I'll score one eventually...
It seems to be such a great cab design, I really wondered if there were some issue with them that led GK to discontinue production. I certainly can't find any issues with mine, and the fact that people who have them seem to want to hang on to them leads me to believe that discontinuation was sort of a misguided "continuous improvement" thing. Sort of like Fender did in the early 70's with their guitar amps...
Thanks again for your thoughts on the matter.