I have DAC1. AFAIK PRE has stronger (and little bit better sounding) op-amps (LM4562) on the outputs. Not sure about RCA but XLR takes advantage of stronger amps by using different level (dB) dividers lowering output impedance. I replaced all op-amps in my DAC1 anyway (LM49860). Better amps give slightly better clarity but original NE5532 is still pretty good. It is very cheap amp because of poor DC spects (that don't count here) but very nice at AC. I use DAC1 driving directly power amp Rowland 102. All my sources (AE, HDTV) are digital.
http://www.stereomojo.com/Benchmark%20DAC%201%20Pre%20Review/BenchmarkDAC1PreReview.htm
http://www.stereomojo.com/Benchmark%20DAC%201%20Pre%20Review/BenchmarkDAC1PreReview.htm