When you tried using the main zone pre-outs, it was the main left and right? You sure you weren't using the 7.1 analog inputs in error? Gain was up full on the Crown amp? Odd that there would be nothing, pretty sure the pre-outs stay live at all times but I'm not reading your whole manual
If you only want to use two speakers on the power amp for 2ch listening, you have a 2ch mode for a second set of L/R speakers built in...look at pg 69 of the manual for two sets of L/R speakers (one for 7.1, other for 2ch listening). Use the bigger speakers for 2ch, leave the smaller ones for multich....