[R-390] query re ground loop (OT)

[wli]

I have a strange problem on my stereo. Loud interference-like mid-range pitched (not 60cps) hum when plugged in using 3 wire plug. Turned off all the dimmers in the house: hum persists.

No noise if I use just a 3 to 2 adapter thus ungrounding the amp. Putting the amp on an AC receptacle in another room: no hum. 

The offending receptacle itself is grounded properly insofar as I can tell.

Do I have some sort of ground loop? What is wrong with my thinking?

Thanks for the bandwidth

W. Li