If one wants to go down the DIY Route, then taking time to learn the methods for testing the Components for their values will be the way to go.
Heat, Humidity and the Breakdown or aging of materials used for the the assembly are all factors usually considered. Even a Solder Joint has a limited life?
Have a look at Tutorials covering Basic Component Fault Finding.
As a strong suggestion and an option that was not available at the time of original ownership, as an addition to any work being undertaken.
Replacing the RCA's on Both Chassis (Phon' and Pre'), as well as the Cable Terminations, to a Low Eddy Design, is an expense will worthwhile of incurring. If done as a DIY activity the cost is not too much.
Additionally using a PC Triple C wire for the Signal Path within the Phon' can also add real time benefits to the sonic. Add the PC Triple C Wire to the Interconnects using the Low Eddy RCA's and the sonic might just jump to a night / day betterment.
Phon's I have been very used to experiencing and very impressed with, have been taken to another level with the above suggestion adopted for the Signal Path.
My wish today is to hear PC Triple C used for a Cart's Coil and a SUT, I have heard it in use for every other role where Wire is to be used, and its presence is always perceived as a betterment