Hello guys!

Sorry if I've posted in the wrong section.

So, I installed an aftermarket head-unit, amplifier and subwoofer to my B7 A4 (2005, non-BOSE speakers - only the rear speakers are amplified, along with subwoofer)
My rear speakers do not put out as much sound as the front ones, because of the factory amp I guess. As I already have an aftermarket amp (for my aftermarket subwoofer), with 4 channels, is it possible to wire this AM amplifier to the factory speakers? If yes, how?
Or should I just connect the rear speakers directly to the HU (as the front ones are) ?