SQ difference XLR vs RCA ?

Balanced analog cabling and equipment tends to reject noise better, especially when you have cable lengths longer than 3 feet. All wiring (AC and audio) can act as an antenna, picking up RFI and EMI noise along the way. You should never utilize an unbalanced single ended cable longer than 3 feet. To prevent noise pickup, RCA cables should be kept as short as possible. I also think that when it comes to the quality of the cable itself, I don't hear as much of an audio performance difference using different balanced XLR type cables, as I do when using various unbalanced single ended cables. To me, the most dramatic difference I've ever heard in my sound system(s) is when I've auditioned and used different RCA cables. The "wow" factor really came in to play then! My systems using a combination of both balanced XLR and RCA type cabling and connections.   

Wonder how many of the responders on this thread actually HAVE equipment with balanced/XLR connections and have tried them.

Well, on my big Mac amp the extra gain of the XLR circuit raised my idle hiss to an audible level. I had to pad that down to remove the hiss. So in my experience XLRs can work in some installations and not work so well in others. 

Well, on my big Mac amp the extra gain of the XLR circuit raised my idle hiss to an audible level. I had to pad that down to remove the hiss. So in my experience XLRs can work in some installations and not work so well in others.

Usually the XLR side, on amplifiers, has a gain around 20 and the RCA side around 26dB.

And the on the preamp side the XLRs are 6dB hotter coming out.

Which makes this an outlier.

Did it hiss even with the XPRs unplugged or the preamp turned off?

I can only speak for my experience. I have a Denafrips stack DAC, Pre-Amp, and amplifier that are all true balanced designs throughout.....I have Morrow Audio MA4 RCA and XLR cables.....same cables, different connections.

My Denafrips Thallo amplifier only accepts XLR inputs, so have only used XLR connections. I did compare the RCA and XLR cables from the Pontus II DAC to the Hades Pre-Amp, and the XLR connections were clearly superior from the very beginning, so much so that I didn't need more than half an hour or so before reverting back to the XLRs.

My assertion is that balanced designs should not be degraded by using an unbalanced RCA connection

I have gone from using RCA to XLR and back over the 40+ years of equipment/listening.  I've stayed with RCA for the last 15 of those years even with 5-meter runs of interconnects from my preamps to the amps.  While I definitely agree about CMMR with XLR, I believe if your system is clean and relatively noise-free, my experience is that RCA is a touch better.  If you have really good RCA or single-ended vs. balanced (XLR) equipment, the mfg has put all the money into single path amplification of the signal.  Even with extremely well spec'd balanced circuits, there are two active components each touching the + and - signals and in some cases even high precision component differences can result in slight sonic differences.  In my experience, the least number of active components "touching" the signal will yield the cleanest signal.  Now if you do choose balanced and XLR equipment, make sure your equipment is balanced through the unit input to output.  If it's a single-ended device with XLR inputs or outputs, you're only going to get a single-ended input signal and output signal.  No CMMR benefit !