Just an FYI, the PS Audio S300 amp is a "true" or "fully" balanced design. I have also noted that when connected to a "fully" balanced preamp, it sounds better with XLR cables.
XLR vs RCA
I recently purchased a new Streamer/Pre/Dac (Linn ADSM/3 w/Katalyst) and a PS Audio S300 power amp. I hooked them up using some high-end RCA cables and they sounded great. After doing a little reading and from advice for PS Audio I orders some XLR cables (Mogami). They are decent but not overly expensive cables. They arrived today so I got it all changed over. I gotta say it is a significant difference. I realize they have a higher gain, but I am amazed about the change in the soundstage. It just blossomed. One of my favorite albums is Aaron Neville - The Soul Classics. I have played this hundreds of times, so I am very familiar with it. Today I heard in a whole different light. Awesome.