So much for stopping the development!
Anyway, I did 3 mods to the setup today:
First one was a fuse in each HT line after the last power supply cap. I have, at the moment, 500mA slow-blow fuses in the fuse holders. The 13E1s idle at around 450mA, so If anything steps out of line, the problem will be terminated straight away. If I get nuisance blowing, with nothing wrong I'll just up them to 750mA. There were no ill effects on the sound that I could hear, so HT line fuses will do perfectly well for me on the final build.
Second mod was to do something I had forgotten to do. Nick's advice to Mick, regarding bypass caps on his thread, prompted me to fit a 1uF LCR 630V polyprop cap to ground, right at the plate connection on each 13E1 tube socket. This gave a surprisingly good improvement to the top end smoothness and detail. It was a bigger improvement than I remember the NVA clone giving, when I bypassed its caps, but then the 10,000uF PSU caps in that amp were of far better quality in the first place, than the 2200uF Panasonics I dug out for this OTL bitza.
Last mod was to replace the nonedescript 1000uF cathode bypass caps on the driver stage, with some nice 100uF, 50V Cerafines I scrounged out of the bottom of a box full of light blue LCR Maplin motor run caps. These cleared a slight fog at the bass end, so again a decent afternoon's work. I now think I've done as much as humanly possible, to neutralise the effects of any catastrophic fault condition that might crop up, whilst not compromising the excellent sound quality too much.
Sgt. Baker started talkin’ with a Bullhorn in his hand.