Is Eurotubes the site in question? They have good prices and selection, but their technical advice is often a bit dodgy and/or warranty-voiding.
FWIW, I've put JJ 6V6s in an Archon 50 (i.e. not an MT15 but close) before. It works -- as in, it makes noise and nothing melts, catches on fire, or explodes -- because the JJ6V6s have voltage ratings more like a 6L6, but it also cuts quite a bit of volume and bass.
I am looking to swap in some EL34s for my MT15. I have never set the bias on an amp, but I understand where the multi-meter comes into play. Could you describe the process for this amp, and where you set the bias for the EL34s?
Measure the voltage from bias to gnd. It will give you a voltage that corresponds to current. 1mV = 1mA. Set the new tubes to the same amount. Just as a guess I expect you’ll see something in the range of 20mV to 30mV.
To all. Not knowing the physical circuit, I would not put EL-34s in there. The heater current is quite a bit higher for an El-34 than a 6L6. You may get by for a bit, but, I would call Mat King at PRS.
That said, 6V6 or 6CA7s should work as a lot of the newer versions have better tolerances.