Low hum from amp

per noromance, more often than not it's transformer hum caused by the transformer coming loose with time--you can try screwing it back down tight, but make sure you leave unplugged for a couple of days before you try it

Thanks to all of you for your response-I appreciate it!
Mike, yes it's coming from both the amp and the speakers but it's definitely starting with the amp.
Thanks again!

Ground loop, maybe.

"the hum does not get louder but stays at the same pitch."
It stays at the same "amplitude" or level (or volume or loudness) not "pitch".
Pitch is different than loudness.

When did this start?