This one has me puzzled... I can plug my SansAmp RBI into my effects return on my Ampeg and it sounds great and there is no unwanted noise. But when I plug the SansAmp into the power amp input on the Ampeg (where apparently it should be plugged into), I get fairly loud hum. Anyone have ideas as to what causes this? I though it might be cheap cables, but if that was the case, it would be noisy even through the effects return. Help!