Bad input socket, is there a generic problem with these?
Of course I thought it was a PbF problem but it escaped that , leaded solder
inside. Solder joints seem fine and no broken tag. Socket make not seen ,
not desoldered yet, maybe make underneath.
The jack centre line is off-centre of the socket centre line , the sleeve
contact is a barrel which is chromed and only makes contact with the now
tarnished ground contact if the metal bush nut is tight and the 4 pointed
grounding tangs are not bent and are making good contact with chassis metal.
So 2 ways of loosing ground contact. With no bush nut the chromed barrel
will rotate a few degrees and then just touching contact to the socket
ground terminal .
Of course I have the preamp out of the chassis now and so do not know what
the initial state was other than the bush nut was not loose when received.
All compounded by gallons of WD40 squirted everywhere that has certainly
lost the tip and ring bypass contacts to ground with no jack inserted.
I can see repeated tightening/ overtightening? of the metal bush nut
plenty of spanner/wrench force can be applied as metal nut and metal bush)
will deform the 4 tangs/ dig deeper pits into the front panel metal, until
eventually no contact.

An aside , why 2x 10 inch speakers for a bass amp?