I owned five Engls in the past. I would not want to work on the PCB's which are thin.The traces are very fragile and you will more than likely have an issue trying to install the resistors needed for EL34 change. If you are willing to snip out the old parts leaving the legs sticking up and solder the new parts in that way it's do able but you will hurt resale value considerably. I have read even Engl has some trouble with the boards and it was not uncommon to see some traces bypassed from the factory. Now this was awhile back. I do not know if anything changed