I love passive subs. My dual RBH EX-1010N are passive. My new dual Funk Audio 18.0 subs will also be passive. You have to hook them up exactly like regular speakers.
So you will need amps like the $300 Crown XLS1000 or $350 XLS1500. Hook the AVR sub output to the RCA inputs of the Crown amp. Then hook regular speaker wires from amp to subs. Control the volume of the amp and sub channel level in AVR.