First disconnect the amp right after the rectifier diodes and before the first filter cap. If either of the diodes isn't working it could harm the caps. Then carefully measure from ground to each of the PT's two main secondary wires (ie not the centre tap) for AC voltage. with AC it doesn't matter which lead is to ground. For DC it's easier if you put the black lead to ground/chassis. If you can lift the ends of the diodes (with the amp powered off) you should be able to use your meter's ohms or diode test setting to check if the rectifier diodes are OK. They should conduct in one direction but not the other.
vortexx wrote:
I did notice that one of the diodes overheated when I was in there with the soldering gun ...
Soldering guns are usually too hot for soldering semiconductor devices, like diodes and FETs. I would instead recommend a smallish 40W soldering iron.