Hmmm... That is odd. Before you write off the Onkyo's pre-amp outputs, though, I'll toss out another test that you might try: hook a pair of computer speakers up to the pre-amp outputs and turn their volume up fairly high (maybe half way or a bit past that). I did this one time with a friend's receiver that I was tinkering with, before any speakers were on hand, and actually used a couple spare sets of PC speakers to putter around with. Sonically, it's nothing that I'd speak highly of, but it would be a second test to find out if the pre-amp outputs are useable.
Well, yeah, sure - take the easy way out!