Was a pleasure.  The 2 wires were the AC heater connections to the valves.  AC will happily conduct through a weak or even non-contact, capacitive gap (which is like a resistor to AC).  But with the earth connection on the wrong side of the non-soldered joints, the heaters were not earthed so the AC would couple into the signals.  Was great meeting you, and relieved that you do take mains safety seriously.