loose tubes

As long as the pins are making good contact and the tubes do not "vibrate" out, it should be fine (SEE DISCLAIMER BELOW). I do the same thing to the tubes in my Audion Silver Night (both signal and output) and use the tip of a thin fingernail file (power is off) to gap them so that they do not ride too high. Since you mention it, I am just telling you that you are not alone in your observation, but I do not recommend this to people in the event that a tube will vibrate out and cause damage to their equipment (my tube sockets are in good condition and grip very tightly, so I don't worry about it). I had thought of using a thin Sorbothane pad (with holes for the pins) between the tubes and the sockets (thought that this might act as kind of a damper), but have not yet tried it out and am guessing that Sorbothane does not conduct current which would have to be verified first.

This is truly fascinating. Has anyone an explanation for this phaenomenon?

I recently had a loose connection in one of my output tubes in one of my monoblocks and it arced and shorted out some equipment upstream from it. The fellow who repaired my equipment thought it was the loose connection that caused it.(I recently moved the monoblock when I repositioned my speakers). As a long time tube person I know that it could of arced anyway but now I always make sure my output tubes are fully seated.