Besides clipping, there are two more reasons why he's possibly seeing 59.xvAC
He's wired at a nominal 4ohm load.
He THINKS that resistance and impedance are the same value.. Obviously they are not.
According to the spec, the AC rail will never change with impedance.. Let's assume this isn't true with real high impedance.
Wired at 4ohm, he's probably rising around 10-14ohms on a tone.
Given that amount, the AC rail has no where near the resistance to fight with the current load, hence the rail will go way up.
Another thing is their is there is something inside causing a voltage leak and the user is just measuring it.
Clamp the DC side and see how much power it's pulling.. Bet it's about 20A or so instead of rhe assumed ~90A.
OP, i bet your DC voltage stays pretty solid at 14.4v, right?
If so, just more circumstantial proof the amplifier is not putting out what you think it is.