Bi-amping...help appreciated

You need to be very careful about the receiver and amp having different gains and power curves. You may be able to fix a small difference with the balance control. My idea, if I understand this correctly, would be to run all 4 channels to the "bi-amp capable" speakers. That is to say, use the outboard amp to run the lower 1/2 of the speakers, and the rec'r to run the top (or vice versa). You could also run 1 speaker (left) with both channels from the amp and the other (right) speaker with both channels from the rec'r. This will basically be similar to bi-wiring, or jumping, the speakers to allow the rec'r and amp to be monoblocked. Both of these configurations will allow you to use the balance control, if necessary. FYI: I asume swithching them to mono allows them to run in "push-pull" mode, which will give them better control over the speakers (more acccurate), whereas using all 4 channels seperately will provide more dynamic headroom. Anyway, it might be fun to play around. Perhaps the mono configuration will allow you to use 1 less set of IC's and speaker cables.

Oops, I missed one of your questions. You will NOT want to run your speaker level out back into the input of the receiver. If you have the cable available, you may want to just run 1 pair from the rec'r speaker level out to one of the speakers and then run both rec'r preamp outs to the amp, and then the amp speaker level outs to the other speaker

You're in luck! Both of the pieces are NAD. There is extensive documentation from NAD on doing this. It is discussed in their sales literature, but not in detail. They do recommend it as the logical upgrade path. While I do have an NAD amp, I've never needed to do it. But, just check with NAD or your dealer. You'll get the answers you need.