Thanks for the suggestions, but I think I found the problem. I'm certainly going to re-tube this guy soon, as some new 12AX7s to replace the 12AU7s would be a good idea anyways.
The tube socket for the GZ34 was the culprit (at least upon initial listening after the fix). The half-sleeves that contact the pins had been spread apart and split over the years, and as the preamp heated, this caused the contacts to spread even farther apart, and were making insufficient contact with the tube.
Buconero117: I won't be "dumping" it because I spent hard-earned cash on it and enjoy vintage gear. Fifty dollars in tubes would be far cheaper than a new preamp. I respect the opinion, but not posting at all would have been more help.