Hook the the receiver directly to the amp bypassing the ---- all together. Hum still there?
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You may have a ground loop. If possible, try plugging the receiver and pre-amp into the same electrical out. If you happen to have a power strip, power filter, etc., with multiple A/C sockets, try plugging everything into it so everything is on the same circuit. If the hum goes away, you can start investigating why you have a ground loop.