d) at least one connector must safely take 360Vdc (equals 250v AC
rating).

Morning Andy.
That's odd. Most connectors and relays have a much *lower*
rating at DC than AC.

I was just doing a standard calculation as for capacitors - usually
about 1.4 to 1. Why is it different with connectors?

I picked a nickel plated NATO style connector These are available
in 9,12,16,21,26 way. The working voltage decreases as the density of
pins increases. RS part number for the 9 pin male plug is 234-685.
They cost about Euro 17. The matching female connector for the PSU end,
and also the cable connectors are
about the same price. So a complete set, of chassis and cable
connectors
comes out at about Euro 70. Both the Hirschman and the Amphenol NATO
connectors are much cheaper at about Euro 8 for the 8 pin. They are
either grey or olive drab. That is not a lot of money for such a good
connector. (Euro 32 for a
set)

I'll look again at Amphenol and the above - they looked expensive in
the Farnell catalogue. 70 euros is quite a lot, 32 is better. The
Speakon 8 way comes in cheaper and is more available, plus Dave made
the excellent point it's shrouded and so no danger on the amplifier end
from B+ caps. Doesn't matter with a preamp, but worth considering with
a three stage amp where to put the smoothing.

I have an uneasy feeling that the D connector female is not
sufficiently touch-proof for safe use above 70VDC.

It wouldn't have to be - it would only need to supply low voltage low
current filaments. The bigger plug would do the B+. Both connections
would have to be earthed. And, as Iain says the one with the B+ would
have to be screened.