[Woolz]

In the AVA problem finding chart it says if you disconnect the amp from the preamp and the noise is still there the amp has a problem, probably in the power supply.  Could it instead be a ground loop problem?

[Speedskater]

No, 'ground loops' require multiple paths for the noise or your signal to flow.  If the only unit plugged into AC power is the amp and the only other cable is the speaker cable then no ground loops.

If it hums under this condition and you make a shorting input plug and it still hums then it must be:
a] bad power supply
b] broken intern connection
c] defective internal part

[Woolz]

Thanks for the info, how do I make a shorting input plug?

[Speedskater]

Find a give-away RCA interconnect, cut it to about a 6 inch length, strip back an inch or more of the insulation.  Twist the bare copper center strand together with the shield.  The connection only needs to last a few minutes.