All things being equal (hypothetical) which do you prefer, XLR or RCA?

I have recently started using the XLR version of Van den Hul The Second instead of the RCA version (both one metre lengths) between pre-amp and power amp, and have been pleased with the outcome. Not an earth shattering improvement, but to my ears there has been a subtle change for the better.

I agree with dave_b. I have even noticed a big difference using an XLR  jumper from RCA too XLR.

i agree that for some, long runs across areas with strong sources of interference will show the benefits of xlr connections and cable, but i suspect that in just as many (if not more) cases the apparent sonic ’benefit’ is due to the balanced source delivering twice the voltage swing to the receiving component

lots of people fail to properly compensate for volume differences and are swayed by the subtly (or not so subtle) louder version they hear

in any a-b comparison loudness compensation is absolutely critical -- our ears are impressed by 'louder', even by fractions of a decibel

If you have a preamp that takes both balanced and single ended devices and a source that supplies both switch back and forth between say three foot balanced cables and three foot single ended ones correcting for any volume differences. See if you can really tell the difference. I just did this experiment with digital cables comparing AES, SPDIF and Optical types. No difference at all over short distances. 

@jjss49 good point about the loudness perception. I actually fell for this way back in the day comparing Definitive Technology speakers vs PSB's. The Definitive's being bipolar (and more efficient at the time) seemed to completely overwhelm the PSB's. However I soon realized after I brought the Definitive's home that they weren't quite as detailed and did not have that razor sharp image that I was looking for for music, whereas as it turned out the PSB's did. The DT's were great for HT but not as great for being able to close your eyes and actually see the band playing in front of you while spinning a cd...I didn't keep the DT's for very long and never looked back...