Onkyo 605, bass shakers, bi-amp?

[Rustmonkey]

Ok - I currently have my Onkyo 605 setup in a 7.2 setup - I recently picked up a pair of bass shakers that I would like to add to the system - now I know I can simply pick up another amp, run the LFE into it and use that to power my bass shakers, but I'm curious to know if it would be acceptable to use the bi-amp feature of the reciever and run the bass shakers off of the rear surrounds - this would effectively make the reciever think it has a 5.1 setup (I would lose my 7.1 rears) with bi-amped from speakers (I believe) - I would then run my front towers off of the front input and my bass shakers off of the surround back input. Does this sould acceptable to everyone else? Seeing as how the Onkyo doesn't matrix hd sound and we watch mostly bluray anymore, I'm not losing a whole lot until I pick up a second amp/reciever to power the bass shakers. If anyone has used the bi-amp feature of the 605, can you still independantly adjust the frequency response for the front and surround back outputs? THANKS!