I think the biggest thing that makes it necessary is modern switching power supplies (the word 'switching' is redundant here) that have a filter block (needed to reduce RF emissions) at the power input, with three line-rated capacitors between the hot, neutral, AND ground connections. The one between hot and ground puts a 60Hz current through the ground wire at a level that's a lot higher than it would be just from stray capacitance. Balanced 120V puts half the voltage from the (formerly) "hot" on that capacitor, and half the voltage with the opposite polarity on the (formerly) "neutral" capacitor between neutral and ground, thus the currents (mostly) cancel out (depending on how close in value the two capacitors are).Recording Engineer said:By the way, last I spoke with Bill, a couple years ago, if I’m recalling correctly (and I very well may not be), he said he’s not sure how necessary the hoopla and trouble is to have 120 balanced on the secondary; sounded like he needed to do more testing there.
Of course, older/classic guitar amplifiers are different, and the "ground reverse" switch on them won't make much if any difference in hum on a balanced power line, as either way its ground/chassis is capacitively coupled to about 60V. Ironically, these "need" unbalanced power.
On the other hand, for best safety, their power connections should be rewired to modern standards. Is this usually done with older guitar amplifiers?