(1) I doubt it was the tubes in the first place. I think by removing and replacing the tubes with new ones, you inadvertently corrected a poor pin contact or something like that. It would be an interesting experiment to re-install the old tubes and see what happens. Very remotely possible that one of the 300Bs was arc-ing over at start-up, or it was something in your power supply. Hard to guess without seeing a schematic.
(2) Tubes don't leak DC. Capacitors do. Anyway, your output transformer pretty much protects your speakers from DC.
(3) Could it have damaged your speakers? If it did not damage them already, then repetition of the act would not damage them. If the amplitude of the "blow" were to increase sufficiently, perhaps it might damage your speaker eventually.
(2) Tubes don't leak DC. Capacitors do. Anyway, your output transformer pretty much protects your speakers from DC.
(3) Could it have damaged your speakers? If it did not damage them already, then repetition of the act would not damage them. If the amplitude of the "blow" were to increase sufficiently, perhaps it might damage your speaker eventually.
