I have an old passive sub made by Mission, that takes the left and right channels filters out the low notes and has a left and right out put for the normal speakers. It think this was quite common before active subs came along. This sub has no mains power and is just a speaker in a box.

I have to say the results are not very good, and it needs lots of power from my amp to work.

The thing that strikes me as strange about the setup you have there is that I didn't think the rears were full range, so I wonder if there is some sort of none standard arrangement going on there ?

Cheers

Cris.