If it is ground loop or volume/gain related, would the noise be initially absent and then get noticeable over time without any changes? That's the symptom that is making me think it's internal as the tube and other components warm up. In the event it is a ground loop issue with the symptom I described, which ground loop isolator with XLR connections do you recommend? Thanks.

Dan - thanks for the contact. I'm crossing my fingers that it can be resolved without costly repair.