as for rocking, the reason that I wasn't concerned in this particular case is what I mentioned in post #2: the low ceiling joists could offer a method for bracing the cab. but then it was pointed out that the cabs could be bolted to the wall. so long as the wall is quite sturdy, that should solve that.
anyway, a question was asked about reconfiguring for two cabs again, with some additional limitations.
these would be about 38" tall, 32" wide, 22" deep. 3" slot port that is 40" long (one brace down the middle). net cab volume roughly 11 and change. tuning roughly 15-16hz.