Hum from speakers

Hello all.  I have a 10 year old EAR 890 amp.  Recently, I've noticed a low hum or buzz coming from my speaker tweeters.  I don't notice it when playing music, but it is annoying nevertheless.  It wasn't there before.  Only recent change is I had my dac, a Bricasti M1, upgraded by Bricasti to the new MDX board.  Any ideas what is going on?
Disconnect the DAC. Noise gone? It's the DAC. Noise still there? It's the amp.
A hum can be a ground loop. Are all components plugged into the same outlet. If not try that.

is it a hum or buzz... the imply different things.
So when I turn on the amps only, not a sound.  When I turn on the preamp, but keep it muted, I hear a hum in both speakers.  It’s a hum with a slight buzz.  When I unmute the preamp, I have a hiss over the hum.  Is it the preamp or something before it in the chain?The tubes in the preamp are over 10 years old.