Power amp humming - help please

OK--typically it's a hexagonal nut in the middle of the top of the transformer,

Thanks again Johnson...

I have two of the Krell power amps (for home theatre) and they both make hums when I switch out and connect the same way, with the same cables.

Does that mean anything? Perhaps cables or something? (I am so ignorant when it comes to electronics)

If the hum is physically coming from the amp, specifically from the power supply transformer, that is DC offset hum. This is eliminated either with something like a Humbuster power strip, or by trial and error to find what is plugged in or wired that is causing the offset. 

If the hum is heard to be coming out of the speakers then this is ground loop hum. 

So as usual we start with precisely identifying exactly what it is we are talking about.

Gong, is the hum coming from the speakers or the amp directly? If it is coming from the amp directly then I agree, it is most likely the transformer. If it is coming from the speaker then you probably have a ground loop. Try putting a ground "cheater" on the amp's power cord and see what happens.