That still may not be enough. You may well find that you are required

to have no DC at all on the chassis connector pins when the cable
connector is removed.

that would indicate all smoothing in the PSU box. Can be done.

Power amps with separate psu are quite unusual these days. I have a
feeling that the safety regs might well be the reason. Solutions that
were
common place in the 60s and 70s, like for instance the octal connector
between psu, amp and preamp, are now strictly forbidden, as are fixed
mains cables, and any other mains connector except the IEC type (with
the possible exception of the Speakon, I am not sure) You have to be
very wary. If your neighbour's world champion, best of breed
Rottweiler bites through your DC cable, it could cost you a pile of the
elusive spondoolicks.

Two monobloks is looking like the easy solution. I have all the
amplification and PSU stages on seperate top plates, so can plug them
into any chassis - stereo, mono. whatever. I was thinking of a seperate
PSU for development purposes, to try out different valve circuits, but
I can do that in-house and then just construct the final units as
monobloks. Not sure what to do about the preamp - that's an easier
situation as you found.