I am not very good at the maths bit, but I think if the amp is supposed to draw 600 watts at 240v this should work out at 2.5amp.
If I raise the mains voltage slowly with a variac , I am only putting 120 volts in when I hit 2 amps. This is with all the transformer output fuses removed so none of the PCB's are connected to the PSU.

This is an AC related problem, in an AC system one can indeed see large currents but actually very little power consumption. The current through the winding is out of phase with the voltage across that winding. So just because the DC resistance is 1 Ohm, you cannot infer that the power simply V * I. In an inductor with an applied AC voltage, the current through the winding at the instant the voltage peaks, is in fact close to zero and so is the power - at that instant.



I suspect that if there really were any kind of short circuit then it would have caused a serious failure already too.

 

Test for shorted output power transistors in both left and right channels.

Yes, you are spot on.
Pictures .........
1 closing the stable door after the horse etc etc. this is what I planned on building years ago. Slightly overkill on the meters but I had them to hand.
2 very easy bit of kit work on. remove a few screws, pull of a few push on connectors and the amp unit comes off.
3 the offending transistors. This amp has very dirty transistors that are all blown. The other amp is much cleaner and all of them are fine. I know that dirt cannot cause this problem but tis odd that corrosion only picked up on the left channel.