Hello, Richard!
That's a really generous offer. Thank you.
Unfortunately I'm no longer in Thüringen (I spent a few months near Mühlhausen in Thüringen a few years ago, great for a Bach enthusiast), the legend is just there because of the avatar picture of the Bachhaus in Eisenach (user name); I'm in Herefordshire, just under two hours from One Thing in Coventry from whom I bought them 6 or 7 years ago, so they're the obvious first port of call for repairs.
It's definitely a fizz and reduced volume on the one speaker, but strangely it only seems to happen for a few seconds on and off for about five minutes after I've been listening for 50-60 minutes. After that it behaves again. I found someone on a website somewhere (Audiogon?) who reckoned that he'd cured a similar problem, caused by dust he assumed, by giving the speaker a few whacks of the side! So, I tried that when I switched off this evening and we'll see what happens tomorrow.
The 1000uf thing is intriguing; I've not seen it mentioned elsewhere.
True to box-swapping form, though (I thought I'd banished it!), a problem is an upgrade opportunity, and I have been wondering about 2805s, which some claim to be a significant step forward from the 63s, although others reckon the Chinese build is less reliable.
First world dilemmas!