I have spent far too much time testing out balanced vs. unbalanced interconnects in many systems.
In my case, it has been one of the only cases in audio where results have been consistent and repeatable. In every case, balanced connections have made an audible difference. The degree of audibility will differ with the listener, but it is there.
James mentioned that the balanced rejection of electronic and RF interference can result in an unmasking of subtle inner details and low-level information. This has been my experience each time as well. This includes interconnect lengths of at most 20 feet, and even with lengths as short as only 1 foot.
The noise-cancelling feature of balanced operation is well documented. An equally important benefit of balanced operation in my view, is the avoidance of ground loop issues (that can also mask suble low-level audible information - ground loops don't always cause hum/buzz!). Balanced interconnects include the "ground" line separate from the "pos" and "neg" signal lines, allowing the pos and neg lines to have matching impedance (balanced). Interchassis ground that could otherwise ride with the audio signal on an unbalanced line (resulting in a corresponding voltage drop etc) instead rides the isolated ground line in the interconnect, which is hopefully tied to the chassis of each component in the chain.
With interconnect lengths under 15-20 feet or so: I feel this virtual elimination of ground loops riding the audio signal lines, is as much a reason from superior audio performance from balanced connections, as is the noise-cancellation from the differential operation. Just my own speculation.
