In case anyone is interested, I thought I'd post an update... I've been giving the AD30VT a workout at times since, and I think I narrowed the issue down to the input jack connection. The cables I'm using have a gold coating on the plug and although I have not removed/re-inserted the plug in the amp all that often, there is some wear on the coating of the jack. Those same cables on the pedal and guitar ends have no wear at all.
Oh, and it is a straight plug so I think there was maybe an intermittent connection issue - maybe worse at loud volumes due to vibration. Posts on Valvetronix and Patchtronix indicated others having such connection issues. I first "braced the straight plug better so there was no side pulling tension, and finally switched to a right-angle plug, and have not had an issue since.