There are advantages to the AES48 standard, but I don't think it's accurate to say that only amplifiers meeting the standard are "truly balanced."
By way of example, most (if not all) ARC amplifiers are differentially balanced yet don't comply with AES48. To claim that all ARC did to achieve balanced outputs is "add a 2nd output ... out of phase with the first" really doesn't accurately describe the amplifier's circuit.
@cleeds You're correct. However I never said that an amp or preamp that didn't meet AES48 couldn't be balanced. In high end audio, there's quite a lot of true balanced product that does not support the standard. Nor did I say anything about ARC doing what you mention above, although in at least one case that is precisely what they did.
In the quote above there is a bit of nuance. When you have two outputs that are simply inverted WRT each other, one output might be ever so slightly different amplitude or phase shifted WRT to the other. In a proper balanced line cable, this can't happen. This might be easier to understand if you think about a simple output transformer with only 2 output wires, driving pin 2 and pin 3 with no connection at all to pin 1 (which is chassis ground).
@macg19 Without knowing more about the circuit I can't say. However the doubling of output voltage suggests that it does not support AES48.