balanced vs. unbalanced sound quality

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!!

I think it depends on your system. 
I have a Spectral system and it was highly recommended that I use single ended. Professor Keith Johnson has said that the signal travels twice as far in a balanced cable and they present there own set of problems. 
Prior to my Spectral system I used balance cables. 
I do have a set of MIT balanced cables that I have never hooked up to the Spectral system. I will try to hook them up this weekend. I will report back after I have them hooked up.
I do use balanced cables from my JC3 to preamp and will try both types of cables when I hookup my Aesthetix IO. 
Keep listening 
joenies 

Professor Keith Johnson has said that the signal travels twice as far in a balanced cable and they present there own set of problems.

That's pretty funny!

If he really said this it suggests he doesn't know what he's talking about!!!
Look at a single ended cable- the signal travels in the center conductor, and the shield is the return path. In a balanced cable, the signal travels in a twisted pair, and the shield is ignored (not used for a return path). The distances are identical.