Tubes: a dummy at a loss for an explanation

Thought I'd learned something about tubes meanwhile, but I guess I'm still as ignorant as before.

I bought my Audion (by Fischer) SET a couple of weeks ago. It sported 2x Harmonix Gold 300B's and 2x JJ 6922's. The tubes had been selected and biased by the designer/ constructor of the amp himself. Yet, something seemed wrong to me: each time I switched on the amp an enormous blow was heared from the speakers.
I had mentioned this to the dealer who had sold me the amp but since there was no reaction on this particular point and the amp sounded good, I thought it was probably normal after all.

I have now replaced the tubes with 2 EAT 300B's and 2 Tesla E88CC gold grid NOS tubes (EAT/Tesla proved a winning team with my previous Unison amp) and guess what: the speakers remain dead quiet when the amp is switched on.

Can someone explain a dummy what happened? were the tubes leaking DC or something? Would it have harmed my gear, had I continued with the original set-up?

Thanks for any insight!
(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.
To go further than Lewm, replace just the JJ 6922's first and see if the problem returns, then do the same with the 300B's. Lewm, is correct that by removing and inserting tubes you may have corrected it, but to remove the possibility of a single tube being the problem this would help eliminate that.