Good question and one I get asked at least once a day.
Point 1- Cables in an audio system are antennas to the outside world.
Point 2- The advantage of balanced lines in audio systems has to do with "COMMON MODE NOISE REJECTION" Balanced lines have 3 conductors (instead of the 2 in single ended cables, ground and positive - popular name RCA). With Balanced lines there is a Ground a Neutral and Positive so any electrical magnetic energy stricking the interconnect cable (and there is plenty of it out there) gets shunted to ground rather than allowed to enter the audio system through the ground on the 2 wire RCA system.
Point 3 - the down side of balanced lines is you need a sending amplifier
(at the source) and a recieving amplifier at the input. So you have two more amplifier stages in balanced circuits than you have with single-ended circuits like RCA cables There are cheap ways to provide balanced circuits, like transformers and IC chips - Bryston uses fully discrete balanced circuits so the quality of the two amplifier circuits are state of the art.
So after all this I guess my advice is to always use balanced circuits when you have long lengths of interconnect - like from the preamp to the amp, because the longer the length the more antenna. Balanced lines are less likely to provide benifits in short lenghts like from a source component to a preamplifer ( 1 Meter or so) and it could be argued that there is a down side with short lengths because of the 2 additional amplifier stages required.
Hope this helps. I also have a more detailed answer in the Newsletters on the Bryston website.