Balanced cables reject noise better than standard cables; this is why they are almost mandatory on longer runs. What is "longer"? You'll get as many answers as the number of people you ask, and to me, "longer" is anything than about 3 m (6.6 ft). This can be demonstrated quite easily using a microphone - at say 4 m runs (13 ft), standard cable will almost surely produce microphony, whereas a balanced cable will not.
However, regarding the signal transfer, my experience is that balanced is really going overboard unless the input of the subsequent stage is not fully differential, meaning that it requires both polarities at its input for proper operation.
In most cases, an XLR connector is offered, but behind it sit yet more electronics which convert unbalanced to balanced or vice versa. While this is quite possible to do, it is far from easy to do it well, and typical solutions use just one op amp for the job, and that all too often of doubtful sonic quality, but of undoubted economic merits for the manufacturer.
Also, using fully balanced implies a floating ground, and this in itself is VERY hard to do well. I have seen only a few designs which had this done really well, most are for show only and if you dig down deep enough, you might evetually find this to be a simple, old fashoned hoax (specific names ommitted on purpose here).
My experiments, and I readily admit they are far from being conclusive, show that the best method yet is to filter the ground and stay with classic single ended inputs. Noise decreases by 7-9 dB (up to 12 dB in one case), which is more than with balanced, and at zero cost in circuit complexity. This is why I use this approach, but do not claim it to be the ultimate solution.
Some years ago, while I was still working on TV, we compared the sonic results between the two by connecting a Studer CD player to a professional Sony mixer console first with unbalanced and then with balanced. We could switch from one to another with a flick of one switch. After 2-3 hours, 5 of us agreed that we couldn't hear any differences using Klein & Hummel active 3 way speakers (12" bass, 5" dome mid, 3/4" dome tweeter) because we couldn't get anywhere near statistical significance in the blind tests we conducted.
In my view, for home audio, XLR's literally only advantage are the truly superior connectors themselves.
Cheers,
DVV
