Amplifier Hum

some causes are:

Loose or incorrect transformer mounting.

DC voltage on the line. Does the hum vary at different times of the day? Do you hear it through the speakers?

Here's info from people who want to sell you a power conditioner to fix hum: Finding and Fixing Hum
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Might be your preamp. I had amp hum once. I changed preamps and it went away. The hum was only noticable when the preamp was turned off.

I had amp humm and initially thought it was the amp. Turns out, Toroidal (sp??) transformers are prone to this as a result of electrical interference. In my case, it was a halogen lamp on the same circuit as the amp. Turned off the lamp, away went the humm.

My current amps do humm slightly due to design, not a big deal though.

Regards,

Even number of grounds? Dimmer switch on the same circuit?