This solves ALL sub problems.

The coolest sub problem is the rare installation where Speaker B is used in another room, but the sub is wired to the SUB OUT jack. When you turn on Speaker B and turn off Speaker A, your second room sounds very good but your main room has only a sub playing.

Solution? See Spiky's answer.

Well, it solves the problems mentioned in this thread, but creates new ones. Mainly, excess bass coming from the front speakers. Which is a larger calibration issue than setting the mains to small with the sub taking care of everything.