Tube Amp Hum

I have a new VTL ST-150 for about two weeks now. Its breaking in nicely, but one thing I notice with this amp is a slight hum through the midbass drivers. Not audible from the listening position, but there nonetheless. I have the amp on a dedicated 20 amp circuit which was quiet as a mouse with the SS amp that was on it before this amp, which eliminates it being a ground loop. Also, all tubes are all seated properly.

So my question is, is this a normal operating characteristic for a tube amp? If so, does it have a degrading affect on the sound? I would like to hear just pure air when my equipment is on like I had before if possible.

thx!