[bikerhifinut]

BINGO!
Didnt try the 10R but with one side shorted the other input goes "Humphrey" (Hum free).
That's intriguing.
I did try tying the phono socket outers together which went counter intuitively as that seems to create a loop between each channel if you understand me. The result was exactly the same.
Do you mean that because I have in a way, two separate mono amplifiers with their own ground returns to the star earth and their own power supplies individually decoupled at each stage that I inadvertently create a hum loop when I plug the preamp in? In which case it would make sense to common the earth returns after each stage and back to the star tag? But then would there be issues with the individually decoupled HT rails and would it be better to common those too?
So do I tie the preamp and phase splitter earths together and take a single wire back to the star? If that is the answer, I assume the point to tie em together will be at the Earthy end of the tail resistor on the phase splitter valves? Not at the bottom of the input Valves as that would upset the convention on grounding, i.e main PSU closest to the chassis earth, followed by output valves, then LTP and finally preamp.
Just trying to understand the process here. I thought I understood grounding but patently I don't!
By the way the hum is very low level and more or less inaudible from the sofa. Maybe I expect too much but I do know from past experience that it is possible to build a hum free valve power amplifier.
I'm sure the problem is a basic error in my earthing. In a previous iteration this circuit with the same output transformers was buillt as 2 monoblocs and i got them hum free. I may also be too fussy.

Thanks for the insight, now i need to understand whats happening and apply the fix.

A bit masochistic but it is actually part of the pleasure to understand whats happening when a plan goes off piste.

A.