EEman is 100% correct that the key to common mode rejection ratio is having balanced input signal to feed the amplifier, but it is important to realize that the XLR connector itself is more a result of a need to ensure a balanced signal is presented to the amp from the pre-processor/other input devices -- the most common places to find XLR connectors and even balanced 1/4" plugs or jacks is with sound reinforcement amplifiers / musical instruments: http://www.musiciansfriend.com/resources/article/Balanced-Vs.-Unbalanced-I-O--Whats-the-Deal/m710696 http://www.sweetwater.com/insync/common-mode-rejection/

From a technical standpoint induced cable noise is also a problem that folks in the telephone and data world also have long dealt with: http://www.maximintegrated.com/app-notes/index.mvp/id/2045

As to the OP's question about "sound quality" there are legitimate reason to consider how a balanced amplifier is operating vs a single ended design with more than a few folks that believe a simpler unbalanced design is actually a better value...