Sure, active bi-amping is something you could look into, forget passive. But for your set up, perhaps more juice to the terminals would suffice. 2 subs, way out do a single performance wise. The more subs you have, the smoother your bass response becomes. Before I understood that, having four subs was an overkill thought (who'd a thought I have that one). But now, going from two monster subs to four is more of a reality.
Does your current AVR have pre-outs already?
No pre-outs. Before I went on this speaker journey, I thought this AVR did everything but cook dinner. Now I find it just sits on the sofa eating bonbons all day. ]