Years ago I TONS and TONS of A/B comparisons of RCA vs. XLR. XLR won out every time. My favorite all time cables are the original Transparent Reference XLR and speaker cables. End game for me.
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.