XLR and TRS
I have a Harman Kardon AVR 254 with a Zone 2 feature. The AVR has RCA pre-amp outs that can be used to install an external amp to run the Zone 2 speakers. I purchased a used Carver PM 175 amp. The amp has XLR and TRS inputs, but no RCA. I purchased some RCA to TR adapters and put them on an RCA cable. I got signal, but NO low end at all. I realized that I need TRS, not TR. The Carver manual says that "selecting which input to used depends upon the output of the source component", which is obvious, but the manual also describes both balanced and unbalanced cables. I'm lost. I've had no luck finding RCA to TRS-BAL adapters, but there are RCA to XLR adapters. I just don't get the balanced versus unbalanced issue. Help?
Since your preamp has only RCA output (not balanced by definition, cause balanced needs 3 wires / channel), you'll use the Unbalanced input - so your choice of cable should be good (RCA to TS).
I've just checked the operation manual for your amp, and on page 18-19 it seems like the standard unbalanced configuration - ring is connected with sleeve (TS standard), and if you would have used a TRS cable and have the ring wire disconnected you would loose 50% of gain - but that's not the case I suppose.
I would suggest to check your preamp output and configuration (such as EQ for Zone 2). Try using another amp and / or another preamp or audio source for you amp to see where is the issue.