If "no subwoofer" is selected, the Rotel for instance looses the ability (according to their tech person) to select the crossover frequency for the re-directed bass, and the bass is simply re-directed to ALL speakers at a fixed frequency of 80Hz. If you have a single sub connected to the LFE out of your preamp/processor and the subwoofer "on" in the preamp menu, then there's no problem. I'm sure that the manufacturers didn't want to touch the complication of stereo subs since connecting all the components is difficult enough to most people as-is.