If you push speaker button A, button B will pop out...and vice-versa. I just tried it on mine to confirm. I believe the manual is in error - the amp doesn't run speaker set A and B at the same time. One set on A is stereo and one set on B is stereo.
I would like to try and figure out if this is really the case; I have a budget of <150 for a preamp, and with a preamp i should be able to power another pair of speakers right?
Would it be possible to do this? Have a preamp to go with the Audiosource amp and get another pair of speakers?