SQ difference XLR vs RCA ?

I use a short .5m XLR connection from a Macintosh SACD (MCD85) to a Macintosh MA352 IA.  I was originally using the RCA outputs and inputs for SACD/CD.  It was a noticeable improvement in sound quality on many SACD & CDs (not all).

My conclusion is that the XLRs brought out the most in the medium and equipment depending on the quality of medium. 

Just to be an old curmudgeon, RCAs have worked just fine for me over the last 55 years.  

Balanced cables always sound better whether you can hear it or not.

Balance components are basically designed to have one side mirror the other side with respect to the electronics inside the chassis.  When manufactures do this they tend a cheap out because they have to do two systems.  So if you get a RCA only device there's no mirroring of the internals so the belief is that more money spent on the electronics you're buying.  Conrad Johnson is a perfect example, they only do RCA connections. 
 

Based on my experience XLR has always sounded better.  I can't think of a single time when I used XLR's that I wish I had used RCA.