Hum independent of volume can be a sign of old/dry power supply caps. I would, per maxwave advise, take it to a friend and try it there.
Also, since you have balanced connection between pre and amp, check where shield is connected on both ends. It should be connected to shell/chassis and not to pin 1 (that would cause noise currents induced in shield to flow thru analog grounds causing constant hum). This is very common problem (1994 AES paper, Neil Muncy).
Grounding questions
I’ve had an issue with a slight hum through my system for a long time. System consists of Jolida Fusion Preamp and JD1000P amplifier. Both plugged into Futman Elite 15 power conditioner. It’s not dependent on any sources. Hum is there when amp is on.
I have a dedicated line for the outlet to the Furman. Hum does not increase with higher volume.
Maybe it’s an issue with the power amp but it could be an issue with the house wiring. The house is 62 years old and the fuse box is original and not grounded. Before I pay $800 to have it grounded, what do you think is causing the hum? It’s not cable related as I’ve swapped interconnects. The pre and amp are connected by a balanced cord.