@immatthewj No need to give a in depth explanation.
It was a reliability issue
My EE will keep a Tube Designed Device under scrutiny for weeks to ensure the circuit is not compromised.
Even when doing general repairs on any Tube Amp' the device is kept back to thoroughly check for reliability. This approach has lost lots of customers who want the device to be sent for servicing / repair returned within a week of having sent it away.
I recall back in the 90's when Tubes Amp's were becoming quite popular, it was EE's like mine that were the resource selected to repair certain Brands Amp's as the producer did not know how to trouble shoot their faulty circuits.
I know my EE during the 90's has trouble shot a few Brands faulty circuits as a result of receiving Amp's that had multiple failures, usually as a result of extended periods of usage. I also know some of the circuits worked on were fire risks, due to how the Output Valve / Tranxs could be affected. Maybe he has even more of these circuit corrections under his belt in the 00's ? I have not inquired for a long time.