@immatthewj Good advice on the 5AR4s. As long as the power rectifier circuit was well designed any change of tube should have a minimal effect. At that point it should be noted that variations in tube production over the years might also have just as significant an effect, positive or negative. My Sovtek had ~6000+ hours on it, figured getting a Mullard to try couldn't hurt. I will likely buy a backup 5AR4 from new production, unless the Mullard clearly shows it is better.
Yeah, the holes are a bit of an issue. If I did it again I would get a step bit or reamer and practice on some scrap first to make sure it remains perfectly circular and centered. I suppose I could have made a jig to get it on the drill press without total disassembly too. Meh. I had to do some hand filing in the end to get it where I wanted. The soldering is super easy, and if you remove the mounts temporarily on the headphone stage there is enough room to do all the work.
@testpilot I totally agree. The honeywell trim pots are a couple dollars each and are made for ATV controls, among other gross implementations. The Alps you could buy for under $25 with the remote motor. There is a ton of room inside the case for upgrades. It should be an option to go stepped or switched relay.