Turns out, the Weber Copper Top rectifier, one of the wires going into one of the tube base pins was loose. Looking at it closely, the original solder joint looked dry, so I guess over time and heat, it let go.
When my client described the problem, I knew then that the amp had lost B+.
It could have been a simple rectifier replacement, but there was another possibility that could have caused the loss of B+, so we decided it was best to send the unit back to me.
The important thing is that the fault was found and it is up and running again.
So just a general heads up on the Weber Copper Top. I also noticed that it is in fact using 1N4007's and a beefed up thermistor, so Weber must have 'upgraded' the components after some failures of those components.