I am still unclear if your plate voltage reads high when you pull out all the tubes and falls when you put them in.
Is there a feedback loop around your power tubes?
In the event you have toasted a 5y3 on account of my advice
(I assumed If no load, no current, and thus no need for a filter reference to ground) I will gladly refund your dough or replace it with one of my own.
In terms of the speaker I was more concerned that you were operating your amplifier into a speaker that was too high of a wattage (and thus too clean and not lively enough) to do justice to the amp.
Your proposed impedance mismatch may be good, but may just as easily be damaging at high volumes. Without a look at the amp I can't tell. If you are going to mismatch your OPT to speaker impedance do two things. First don't mismatch by more than 100% (no lower than 4 ohms if your builder was telling the truth). Second start quiet and see if anything sounds weird, proceeding slowly to louder planes.
I have mismatched many times with no ill effect, but I play Fenders, a notoriously tolerant of mismatch amp. I have seen several Vox and Marshall amps release their magic smoke on account of an impedance mismatch.
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i just checked that out a second ago...with all the tubes pulled B+ shot up to 380vdc! the transformer is a "chicago standard transformer' which i understand later became staco. it has a nice helpful label on it, and i just noticed it is rated at 40mA. is that too low?
regarding impedance mismatches, i discovered that i had in the past miswired a cab at 32 ohms and had been using it sporadically for about 6 months! i even cranked my reissue marshall thru it. my poor old 4 ohm traynor, too. YIKES. but that to me was a lesson that maybe its ok not to trip out over impedance too much.... :D
one last thing, how do you check filament voltages? my probe to ground readings just now were all only 3.5 vac. could this be my problem finally? bad 6.3 v heater supply? would tubes even glow or operate at 3.5 vac? the 5 v is ok.