I'm not 100% sure, but it wouldn't surprise me if the Onkyo runs in either poweramp or preamp modes and can't do both at the same time. Most likely it detects the super high input impedance and ramps down the power output to protect the output stage, but has the side-effect of starving the surround speakers because you can't run passive speakers on line voltages. Looking at the manual, you might be able to get the effect that you want by hooking the Crown up to the Line Out jacks for Zone Two and running both zones at the same time. The other option would be to get a cheap power amp to run the surrounds.
FWIW, most power amplifiers (or integrated receivers) don't handle getting hooked up as a line-level source very well, so the fact that the Onkyo was able to run itself down from being a 100 watts-per-channel system to keep from blowing up the Crown or the speakers down-stream from the Crown is pretty cool. Anyway, I'd take a look to see what the recommended method is for running your receiver as a preamplifier is. My guess is to hook up the power amp to the Zone Two line-out jacks, but I'm sure people with more experience with that amp than mine (zero) will have some suggestions.
