Just got a new power amp

If there is an audible difference, something is defective.

theoretically balanced cables are for long runs typically found in professional environments in order to reduce noise that could be picked up. Home audio is short runs, and a cleaner environment.

I preferred mine with XLR and the amp was not defective...I was lucky to have identical cables in house in both RCA and XLR, so no ulterior motives to choose one...difference was relatively small...

I think two things are going on.  First, Paul McGowen at PS Audio has always been a fan of using balanced cabling over single ended. Second, and probably the most important reason, is that the S300 is a fully balanced design.  If you use RCA's, then the input has to be converted to a balanced signal first.  Thus adding another layer to the circuit.  

I currently have a S300 amp with the PS Audio Gain Cell preamp in a secondary system.  When I got those two pieces, I did a comparison between using a balanced cable and RCA's (both were Audioquest Columbia's) and the balanced cable did sound better.  Not by much, but it is tighter in the treble and midrange driving my Martin Logan electrostatics.

- Jeff

When single ended rca is compared to balanced XLR, how many people are actually taking voltage measurements to level match each one?