For a start - thanks for the helpful suggestions, much appreciated. In
response to Dave P, not just a one-off so I need something I can
re-order from usual suppliers. Amphenol is nice, of course, but
expensive and nothing on ebay in the UK.

Your point about capacitors in the amp chassis is extremely perceptive.
I hadn't decided how to do this - one B+ supply to the chassis and
further smoothing there or several B+ supplies from the PSU. As you
say, this makes a big difference. Any capacitors in the amp chassis
would rule out XLRs. It would, however be possible to use Speakons. The
8 pin speakon was used by Beard audio as an umbilical, so he obviously
did a bit of research and had that idea. I have to say it's tempting.
Plug is big though. It's again a bit expensive - would be £19.45 for
two sockets and two plugs. Still, could be worse.

4 pin speakons cost £6.63 for two sockets and two plugs. That's very
tempting. Would have to be clearly labelled PSU though.