Let's be clear about XLR & Balanced first.
The XLR is a *type* of connector.
It could be used for almost anything, and has been. It was developed as an audio connector for 3 wire balanced signals.
The RCA is a *type* of connector.
It could be used for almost any use, and has. It was developed as a consumer type coaxial connector.
The reason some RCA plugs and jacks used for *audio* today are more expensive than XLRs (built mostly for pro audio and broadcast) is simple enough. The XLRs are made in very large quantities and not of exotic materials (possibly excepting one XLR intended for high-end audio I saw). Whereas the RCA components you see for high-end audio use are made in relatively small quantities and use exotic metals and/or platings and insulators (teflon for example).
Similarly, the construction and manufacture of finished cables for *high-end audio* is also a low quantity process utilizing high cost components when compared to commercial cables made for the broadcast and pro audio industry, so the prices are higher in general.
The use of *balanced* lines for audio is another matter entirely. The use of balanced lines for audio gives you a reduction in potential noise and hum due to something called CMRR (common mode rejection ratio) which is the cancelling of signals that are impressed upon the wire (picked up) from external sources (hum, noise, RF, etc.).
(CMRR is *not* an attribute of the wire, but of the receiving circuitry - if the circuitry doesn't do this right, or is not balanced electronically, the CMRR degenerates back to no better than single ended, or worse)
The single ended connection does not have this immunity and depends upon sheilding alone. Often this is not a problem since in practice this is a problem under two circumstances:
1. Loooonnnnngggg runs of wire
2. Low levels of signal over longish runs
The second is found with microphones (similar to phono carts in signal levels) to be a significant problem. A typical PA system or studio must because of the physical requirements have very long runs with signals of all levels going back and forth. So runs of as much as a hundred feet or more are not unusual. Contrast that with the 1m run between your CD and preamp!
Studios and PA systems use 600 ohm balanced lines to overcome the problems of hum & noise as well as HF rolloff over a long cable run.
Which is better? In theory the balanced cable is better when driven with a balanced signal and recieved with a balanced input,better yet if it is also 600 ohms (low Z).
In practice this also means *more components* and more "things" for the signal to pass through, so may not actually be better, or worse than the RCA/single ended cable IN THE APPLICATION we are discussing (your system).
_-_-bear
