I don't know specifically about the Onkyo, but in most HT units, what determines the distribution of bass in normal audio to the main and surround speakers is the small/large setting for those speakers, and what the subwoofer yes/no setting controls is what happens to the LFE signal (low frequency effects, which carries the rumble from passing starships and exploding deathstars). With subwoofer set to no, the LFE is mixed into the sound that goes to all the speakers that are set to large; set to yes, and the LFE is directed to the .1 channel whether there's a sub connected to it or not and not to the other speakers,