Sounds like the problem is ground related. I had the same problem with my Krell KSA 250.
Some suggestions
1. If your Preamp is a 2 prong plug style, plug the Preamp and the Amp into different circuits.
2. If the Preamp is a 3 prong use the cheater on the Preamp only leave the Amp with its 3 Prong in tact. Still try different circuits for each.
3.If your Amp has balanced and RCA input and you are using the RCA make sure that the copper jumpers have been inserted into the Balanced inputs.
4.Most hum comes from Video sources try unplugging any RF or Video inputs to your Preamp to see if the hum disapears.
Some suggestions
1. If your Preamp is a 2 prong plug style, plug the Preamp and the Amp into different circuits.
2. If the Preamp is a 3 prong use the cheater on the Preamp only leave the Amp with its 3 Prong in tact. Still try different circuits for each.
3.If your Amp has balanced and RCA input and you are using the RCA make sure that the copper jumpers have been inserted into the Balanced inputs.
4.Most hum comes from Video sources try unplugging any RF or Video inputs to your Preamp to see if the hum disapears.