Well it's been one week on and I am still loving the amp. I've been listening to lots of old classics and rediscovering the pure joy of music.
I couldn't resist building and fitting the speedball upgrade. It's a personality flaw/feature - I can't not have the best possible performance out of something. I can't really say how much the performance improved with the upgrade as I hadn't completely got used to the sound pre-speedball.
Anyway: I have noticed the amp now has a bit of a quirk on startup. I get some very quiet mains hum initially, that then becomes louder (in both channels, though a split second later in the left than in the right). This then gradually recedes into "darkness" and the output is completely silent - and the music then sounds wonderful from then on. This whole warming up period lasts about 30 seconds. If the volume control is at zero or a fraction above, after the hum is gone, I get the hum back. If the amp has been on for a few minutes, then the zero-volume buzz doesn't happen.
It doesn't bother me but I do see it has some correlation with symptoms reported in the 5998 thread. I'm casually wondering whether there's been some sort of component change in the last shipment of speedball that's caused this to happen. I'm happy to take measurements etc if necessary but as I say, it's all working anyway and I love it :-)
Paul.