No sweat on bi-amping. What you have to look for is bridging.
If your speakers are 4ohms, the 906 will see 2ohms load.

Then bridging is not recommended for speakers below 8ohms.
But bi-amping is no problemo amigo. Your 906 will do just fine.

For more information on Bridging, Bi-Amping and Bi-Wiring, check in the
Audioholics section of speakers impedances and matching amplifiers, also
in the section of dito terms.
Hope that clears the sky,
_____ Bob