Going a bit against the grani:
The onkyo processor has the option to re-assign two of the surround outputs for bi-amp ability.
IF the amp allows you to choose frequency as well, so much the better. Anyway, why not borrow a cheap 2 channel amp and try it out. Since you use a 5 channel Rotel, try to find a used 2-channel from the same company.
You will get some sonic improvement which usually translates into slightly "clearer" sound -- i.e. you get the impression there is better separation between instruments and voices. You have more power on tap because you have added two extra channels -- you are just relieving some of the strain put on your present amp...
I used to passively tri-amp simply because I happened to have amps lying around that happened to have identical input sensitivity. The result was not stellar -- but improvement there was.
Of course, as these things go, active bi/ multi amping is in another league altogether!