Low hum from amp

If it's coming from the amp -- the physical amp cabinet -- and not the speakers, it may be the power  transformer humming. This occurs when there's a little DC offset on the line (not uncommon). A cure is available as the AVA HumDinger. I have three; they work great.

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.