I just had a similar problem on new Conrad Johnson Amp/Preamp. Here is the response I got from Spearit Sound eliminated the problem. Hope this helps you.
"Im 99% positive you have a ground loop between the preamp and the power amp. Thank the Underwriting Laboratories for requiring equipment to have the third ground plug. This isnt a problem with less expensive gear because they use all sorts of filters so you dont have any hum. The problem is they tend to kill the sound of the components. cj prefers to do a clean design and solve the problem at the source, the wall outlet. The usual fix is cheater plugs. Try one on either the amp or the preamp and see if the hum goes away. If not, try a cheater on both plugs.
Its possible that the hum is being induced. i.e. a strong emi field near the amp or preamp. It that is the case, and this is rare, it may be necessary to move the components slightly to reduce their susceptibility to emi and rf. Again, I do think the problem is a ground loop."
"Im 99% positive you have a ground loop between the preamp and the power amp. Thank the Underwriting Laboratories for requiring equipment to have the third ground plug. This isnt a problem with less expensive gear because they use all sorts of filters so you dont have any hum. The problem is they tend to kill the sound of the components. cj prefers to do a clean design and solve the problem at the source, the wall outlet. The usual fix is cheater plugs. Try one on either the amp or the preamp and see if the hum goes away. If not, try a cheater on both plugs.
Its possible that the hum is being induced. i.e. a strong emi field near the amp or preamp. It that is the case, and this is rare, it may be necessary to move the components slightly to reduce their susceptibility to emi and rf. Again, I do think the problem is a ground loop."