While comtemplating on whether to add a stereo amp to my system to biamp my pair of RTi150, I came across this post on another forum. It is a very clever trick to use your unused SB amps in your AVR7200 to biamp your front speakers. Of course, this is assuming you have them free, ie, you are not using a 7.1 setup. This method also works with the Denon 5803 or similar 7.1 receivers with multiroom cabability.
1. Use RCA cables to connect the main L/R pre-outs to an unused input
2. Go into the Multi-Room menu and set Multi-Room to ON
3. Set the Multi-Room source (MULTI IN) to the source you connected the L/R pre-outs to
4. Go to the Advanced menu and set the SB AMPS to MR SP (Multi-Room Speakers)
5. Connect the main L/R amp terminals to the High Freqency inputs on your speakers, and the Surround Back amp terminals to the Low Frequency inputs (and remember to remove the jumper straps!)
From there it's just a matter of setting the Multi-Room volume (MULTI VOL in the Multi-Room menu) to match the main L/R amps. I used an SPL meter to calibrate the HF and LF separately (with each one unplugged alternatively) until they matched in level. Then I connected everything together. You only need to do this calibration once since the preamp will carry the same MASTER volume level.
I just finished adding this tweak to the system. Works great! I also like the flexibility of being able to adjust the level of the HF and LF separately. More power is always better :p