This just in:
I tried this with two amps, my Creek and an old Arcam. Was going to try the receiver unit from my Denon mini but the spring clip speaker outputs put me off.
I set the Yamaha Receiver to have no centre, and hooked up the PMCs at the front and some Diamonds at the rear. Front left and right preouts into a spare input on each amp, played some music and matched levels so that volume was the same whether from the Yamaha or the external amp.
Results - both worked fine. The Creek had to be ramped right up to match levels, presumably because it's a passive preamp and the volume needs to go quite high anyway. The Arcam was lower - 10 o'clock maybe, but then I wasn't messing around with speaker levels on the Yamaha so probably not optimal.
I then plugged a DVD player in, played a movie briefly, and all seemed well. The one thing I'm worried about is that I did not try this with a centre (haven't got one, though I suppose I could put in a diamond), which could skew results considerably, since 70% of what was going to the front pair would go elsewhere - how much this would affect them, I can't say.