Arthur(welcome back), your 3480 doesn't have provision even for the phony "bi-amping" that some of the multi-channel receivers advertise, using the back surround channels. The provision in the 3480 is simply an internal splitter to the same two output channels, allowing two sets of speakers to be driven in parallel. There's no electrical difference regardless of which set(or both)of terminals that you use, and it isn't bi-amping.
Actual bi-amping requires separate amplifiers with an external crossover before each amplifier so that each amplifies only a limited frequency range. The internal speaker crossover has to be bypassed.
Your 3480 has a quite powerful amplifier and it's highly unlikely that you're using its full power capacity.
