Griff, you're not crazy, but whatever you've noticed definitely isn't "bi-amping". The procedure that you followed doesn't double the available power, as your PS implies; the actual power increase is zero. A 7.1 receiver has one amplifier with seven channels of output transistors, not seven separate amplifiers, and you didn't have any "unused" amplifiers. The output transistors in each channel have no power of their own; they act only as valves to let in the required amount of voltage from the one power supply section. Using two sets of "valves"(one previously not connected to a speaker)rather than one can't increase the capacity of the power supply section in any way.
I'll throw out a remote possibility that some of the output transistors for the main channels weren't up to spec and were causing distortion at less than the rated power output. When the output transistors in the back surround channels(up to spec?)were put into use they allowed the rated power to be delivered to the section of the speaker to which they were connected, allowing cleaner reproduction of those frequencies. Again, this is highly unlikely speculation, but if you'd like to try using only the back surround output transistors to see if they alone make a difference, this is easily done. Disconnect the speaker wires from the main output terminals(keeping the receiver in "bi-amp" mode, of course)and replace the connecting links on the speaker terminals so that the the back surround channels(now carrying main channel content, of course)drive both the high and low frequency sections of the speaker.
