I'm no technician, so am not going to make a suggestion other than to find someone qualified to repair this. The voltages involved in valve amps scare me... dunno about you. Any kind of bang or pop or whatever is probably a short in my non-expert opinion, which has blown a valve again by the sounds of it...

You could look at this - and maybe direct some of the guys you've had look at it to the curcuit diagrams & valve biasing information etc.

Also, if you can say where you are, maybe someone can recommend a decent tech to repair it for you....