I pass along his thoughts after asking him to put them in writing:
"I can't tell you what happens metallurgically in the contact zone, but it was received wisdom at Fluke that you never use a gold pin in a base metal socket or vice versa. Tin on tin is good, gold on gold is best, but gold on tin is worse than tin on tin. I heard the same thing when I worked at Intermec.
"The issue seems to be not so much with connectors that are regularly plugged in and out but with permanent connectors and particularly with ICs in sockets. Over time, the base metal would oxidize and make a noisy connection (some of these thin films of base metal oxides and intermetallic compounds even have semiconductor properties).
"Perhaps tin on tin was okay because there were no dissimilar metals with different electromotive potential (obviously a big problem if there's moisture) or maybe it was that the tin was soft enough that the platings on each part actually smooshed together.
"The issue wasn't that the connections totally opened up to where they'd overheat or fail to carry high currents, but that small-signal connections would get noisy. It may well be that in a tube amplifier the thermal cycling creates enough wiping action to keep the connection good, or that the voltages are high enough to destroy oxide films. But if I was designing that sort of thing, to be on the safe side, I'd stick with nickel-plated sockets. Of course it might also be that your "gold" tube sockets have just a thin decorative whiff of gold on them and it wipes right off of the contact area after a couple of insertions, leaving you with the underlying nickel plating at the actual point of contact."
At any rate, food for thought.