The voltage at the cathodes of the preamp tubes is very low, around 1 or 2 volts. So electrolytic capacitors rated at 50 volts would be suitable.
The way I read it, the mods suggested specify what goes on the cathode of V1.
V1 Pin 8 -Townshend/Who CP103 1k5 + 64uF
This looks like it's the existing 1k5 plus a new 64uf in parallel.
V1 Pin 8 -Jimmy Page amp 2k2 + 100uF with additional parallel 1k5 resistor
This looks like it's the existing 1k5 plus a new 2k2 and a 100uF all in parallel. You could use an 890-ohm resistor, but that's not a standard value and even if you got one it would be harder to go back to the original configuration if you don't like the mod.
V1 Pin 8 -SAP DR 103 2k2 + 100uF (no additional resistor)
This looks like the 2k2 replaces the 1k5 and there's a parallel 100uF.
V1 Pin 8 -Late '70s, Late '80s and OL/Lead 1k5 + 150uF
This looks like it's the existing 1k5 plus a parallel 150uF.
The rule of thumb with tube amps is the voltage can vary + or - 10%. If it's 20% over at V4 pin 6 that's starting to get on the high side. The voltage at pin 1 of V4 is good so it's just variance with the tube and not a problem with your circuit. You could try a different 12AT7 there if it bothers you, but the higher voltage isn't exceeding the tube's specs so if it sounds good I wouldn't worry about it.
Thanks for the compliment about my DR504. It was actually my second tube amp project. The first one was a Trinity TC-15 I built about a year earlier, but I had been building electronic projects for over 30 years by that time. The assembly work for the original Hiwatt amps was contracted out to an outfit in England that also did work for the British Navy. The wiring was very neat and close to military specifications. I tried to emulate that as closely as possible when I built mine. Also I started with a fairly big chassis, like the originals had, so it was easier to keep the wiring from looking cluttered.
Here's a picture of the outside of the chassis.
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