Even though the amplifier will be driven balanced on a single-ended source, there are many advantages to using balanced (XLR). This would mainly be beneficial if you can connect from the beginning with XLR all the way to the amplifier.
An RCA connector consists of a + signal, and ground (shield). An XLR connector consists of a + signal, – signal, and ground (shield).
When using RCA, the ground (shield) is connected to the – signal input on the amplifier. If you have any noise or interference in your system, it will be amplified through the amp, and out your speakers. The amplifier then derives the – signal from the + signal by inverting it 180deg out of phase with respect to the + input.
When using XLR the + and – signals are a product of the source, and the ground is simply a shield. The major advantage is that the ground or shield isn’t tied in with any of the signals. Not to mention that neither of the signals are “made up”, or referenced to ground.
All in all, we have found that XLR connections have a much better sonics than RCA. The most noticable would be that the noise level is reduced. Some of the cheapest XLR cables, can be much better than some of the expensive RCA’s, so that would be something to consider. However, if you have a REALLY quiet system (noise), you wouldn’t see much improvement.
Someone asked this question at AVSforum.com
Finally, this is the best short straight forward answer that I can find on the net about XLR Vs. RCA connections. Thanks EJ!