One reason for going with balanced operation is the effect that this has on the interconnects used, rather than just the fact that the amp might have a balanced input.
There is a standard for balanced operation that has been around for decades. It is there so that you can reliably and repeatably set up a system, with expectation for good results, regardless of the length or cost of the cables used.
IOW if the standard is used, the cables in the system cease to have significant effect on the sound of the system.
Here is the standard:
1) Pin 1 is ground. (pin 2 in the US is non-inverting, in Europe it is inverting)
2) The signal shall occur between pin 2 and pin 3 of the XLR. Ground is ignored!
3) The output impedance of the source shall be low enough that driving a low impedance can be done without change in frequency response or distortion. 'Low impedance' is typically less than 2Kohms.
If any of these rules are ignored (which is common-place in high end audio) the result is that the cable will express some artifact, and the length will be a problem. A common myth is that there is no advantage to short balanced runs; the truth is that balanced operation is lower noise even if the cable is only 6 inches long.
The take-away here is that if you can hear a difference with the cables in your system there is a problem. Its very likely that of two cables being compared, one might sound better than the other; the simple fact is that neither is entirely correct. It is the balanced standard that allowed the record labels to do the job that they did back in the late 50s (often known as the Golden Age of Stereo), when exotic high-buck cables did not exist.
The funny thing is that to drive the cable correctly such that its artifact is controlled and prevented is not difficult nor more expensive! The main thing seems to be education on the part of designers who very much seem to me to simply not know about the standard.
This is why I put up my prior post. Of all the preamps mentioned, how many can drive a 2000 ohm load without loss of bass or an audible increase in distortion or overall loss of volume? Its true that many amps have a high input impedance where this might seem to not come into play, but like I said earlier, if you ignore the standard then welcome to audible differences between cables, something the balanced line system is supposed to prevent. I am indeed saying that you need the low output impedance even if you are not driving a low impedance load.
