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?
robal
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.