My thoughts for this involve the high voltage supply to the valves, I would not advise attempting this fix yourself, I would personally get the guy in the guitar shop to do it, maybe he will offer a reduced rate if you point him at a likely problem?
The standby switch in that amp is a dpdt,
The first half switches the LEDs from red to yellow, while the second half drives the input of an opto-coupler which switches off the HV supplies for the valves.
My first thought would be that this opto-coupler is not functioning correctly, so the supply is staying on while the LED is changing.
The opto-coupler is located as follows:
and here is the pin out
Under normal operation of standby pins 4 and 5 would become connected.
AS YOU CAN SEE THESE PINS ARE CONNECTED TO THE HIGH VOLTAGE LINES, I WOULD NOT RECCOMMEND TESTING THIS YOURSELF