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?