I got the cat to do it earlier:
Did the rumble-filter too. Fiddly, but a good learning experience.
The slight burn-mark on the orange wire near the rumble filter was neither the cat nor me! It was there previously, almost certainly done by whoever tried to put the far too big mustard caps in nearby. I’m actually tempted to replace the two largest as they really don’t fit!
One of the black Hunts caps on the treble pot just shattered into a load of plastic shards as soon as I went near it, so I suspect it had been broken for years and may well have been the problem. The others measured around 0.04-0.07uF on the Fluke. The Peak ESR meter wanted nothing to do with any of them. I have a feeling it can’t measure really low values, but they were clearly screwed so no need to overthink it. The fact one was likely not even in one piece was almost certainly the issue. It is certainly sounding better and more balanced now. I’d not say 100% compared to the precision I’m used to with the Audio Synthesis attenuator, but not obviously off anymore and it sounds very nice indeed.
I seem to have lost the light on the front. May just be loose, or I may have trashed the bulb filament fiddling around moving it. No big deal. I’m kind of tempted to leave it as I was a little worried about heat damaging the ancient printing, though it does look cool. That said I will be using the AS Passion long-term, I just wanted to get this working. I’ll test the bulb later. I potentially have a couple more (Point One mono and a tatty mono Varislope).
Pretty sure it is humming less too. Really not an issue at the listening seat, though the JR149s are far from efficient. I’d certainly hear it through the Tannoys! The rumble filter very obviously works now too. It was hard to hear previously. Obviously something to leave switched out, but nice to know it works.