Bridge or bi-amp? Which should I do?

I may be dense (may?) but, how are you intending to do this bi-amping? Do the high-level outputs on the C370 stay active when using the pre-outs? Or does it have main-ins, and you are going to use a Y-splitter off the pre-outs? And if either is true, does the amp have adjustable gain so you can match properly? Doesn't bridging or bi-amping with non-identical amops introduce all manner of potential to screw up something?

I decided to bridge my amps and love the results. Big, open sound, huge soundstage.

Pretty sure they do, as I tested them prior to this response.

NAD amps can be bridged, but when I did so with my 3400 and 2400 amps (sister products) to rum my Mission 765's, there was a very definite decline in the sound quality. The trade-off to maximizing power output(and there is always a trade-off) is the sound output took on a much harder edge. Bi-amping preserves all of the sonic/headroom benefits with absolutely none of the trade-offs.
In your case, I would have used one of the amps (the one with independent level control) for the HF and the other for the low; or the more powerful amp for the LF.