Mixing XLR/RCA Outputs on Marantz AV-8801 Possible?

My B&K 200.7 amplifier lost a surround channel (almost no sound) and it will have to go out for repair. In the meantime, I'm hooking up a spare 5-channel amp and separate 2-channel amp to run the system. My Marantz AV-8801 pre-pro has both XLR and RCA connections for audio out; I've been using the XLRs. The 2-channel amp I'll be using only has RCA inputs, and when I connected it via the RCA outputs on the 8801 and powered everything up, the sound was very faint. I do not have the manual for the pre-pro and thought it was kosher to mix output/input formats as long as each was kept to its own and not duplicated by hooking up both to an input/output. To clarify, sound across all channels was faint, not just the two surround channels connected via RCAs. Anyone ever been down this road?
The RCA/XLR combo has nothing to do with it. It's perfectly fine.
Disable any room tuning or EQ or correction circuits first.

Thanks for the responses...in the meantime I found a neat XLR to RCA cable by Seismic Audio that will let me make the connection to the PS Audio 2-channel amp. That should put things back on solid footing, I hope.
Boy, do I feel dumb (maybe?)…..I loaded my B&K 200.7 amp into the rear of my CRV and positioned it sideways, with the binding-posts facing to the right. Since I'm right-handed, and because I'm OCD, I tested each binding post to see if it was securely tightened down. The third set had one post that was slightly loose, maybe 2 short turns. Would that have been enough to cause the volume problem if it were the post for that surround speaker output?
Try removing everything but L and R.  Play a stereo source. Diagnose that way.