balanced vs. unbalanced sound quality

XLR has always been better sounding for me

I'm currently looking for a cdp that will blend well with my krell kav-300i and vanderteen 2c's. I recently picked up an NAD S500 cd player based on positive reviews and described sound quality. It sounded better with the krell versus other players I have tried (i.e. Rega Apollo). Both the krell and S500 have XLR outputs, so I decided to give it a try. Wow, the difference to my ears was rather substantial. Yes, there was a noticeable increase in volume, but more importantly, there was a more neutral sound to the music. It was much more even sounding, and was exactly what I was looking for. I kept the S500 connected in RCA as well and kept going back and forth, adjusting for volume each time. There was no comparison in this particular setup. The XLR connection was much better and provided a more even presentation of the music. My question to all this is: How is this possible? 

Units that have XLR output/input connectors (even if the internal circuitry is not "balanced" (2 identical audio circuits for the + and - leads)) take advantage of utilizing separate ground paths for each signal. Common ground paths are a huge factor in electronic noise. All currents flowing thru traces or wires drop a voltage in proportion to the current flowing thru it.  Keeping the ground or returns paths as separate as possible reduces the algebraic addition of all noise created by current flow. XLR wiring keeps left and right returns off the noisy ground path. Units with XLR connectors make an effort internally to accomplish that. Again, even if it is not a pure balanced circuit internally. 

I find the quality of interconnect wire to make more of a difference than the connection on the end. Just because a low quality cable has a XLR connectors doesn't make it sound better than a high quality cable with RCA connectors, especially for short runs. That being said, since my present system is fully balanced I do use XLR balanced interconnects. I also prefer the click connection of the XLR over even a locking RCA connection.

In my system balanced was quiter and more dynamic i was running single ended before that.Good luck!!