SQ difference XLR vs RCA ?

Probably not...

Nope, unless you own components that sports balanced design. Components with unbalanced design even with XLR jacks offers no improvements over RCA with short run of Interconnects cables.

When in doubt, check manufacturer specifications :-)

RCA and XLR can be implemented differently.   I think your question assumes that they are the same signal sent through RCA wires or XLR wires.  But XLR can be implemented different ways.  Often a small transformer is used in the XLR input.  This will change SQ, maybe better, maybe worse.  It may pair well with your preamp, it may not.  So implementation methods of XLR may make your question more complicated than you meant and answerers may not realize how their xlr is implemented.

In my amp, Decware, it is hard to compare.  The XLR amps have a transformer that adds about 8dB of gain.  so you have to adjust volume to try to do an A/B. Right now Im using XLR.  However, I've upgraded my RCA connectors coming out of the amp and may go back and forth.  I don't have a strong opinion.

@mabonn,

SQ difference XLR vs RCA ?

See article below:

Mike

http://www.atma-sphere.com/en/resource-why-balanced.html

no, most likely