Quoted from TheFamilyArcade:sometimes I feel I get more time in a mode when i stack it with a mb, but that might be my imagination.
Not your imagination. On virtually every modern Stern, a ball in the pop bumpers will delay or pause any running timer. Even during MB. Often letting a ball intentionally drain during multiball (while the ball saver is running) can be a good idea. On games like TSPP, starting another mode while another mode already running (and multiball) will reset the timer to max. Stacking modes with a multiball is probably generally more profitable than just stacking multiballs. YMWV, of course.
AC/DC has a multiplier similar to BSD, although I haven't haven't had much time on the new software to notice the difference.