I have the AD60VTH (the head version of the 60-watt) as well as an AD50VT 212 and have never had this problem. What you're describing I had never heard of before.
I would post the question on the discussion forum at www.valvetronix.net
That site is THE best resource for these amps and Voxman can be very helpful.
Give it a shot.
Guitars: 2003 and 2004 American series strats, Squier Classic Vibe 50's Strat, Squier Deluxe Strat.
Amps: Line 6 Spider IV 120, Vox AD50VT 212, and Peavey Transtube Bandit 112.
Pedals: Digitech Bad Monkey.