Welcome to the forum!
Fine question.
I'd run a loaded 4X12, no prob. A little sketchy for me in a 2 x 12, but I'm overly protective of vintage gear, and others may offer different advice. I do run 2 30 watt speakers with a JTM45 with no prob. I'm guessing Jim(Scumback) should be able to offer the most experience and advice, but there are definitely others.
As for me, I can't help but err on the side of caution-depending on the vintage of the gear in question. I can tell you from experience, that it sucks ASS blowing original speakers in vintage 50's gear
-and it's definitely affected my bias. I can't bring myself to push more than 5 watts through my 1940's Jensens, despite their 15 watt rating. I'd probably hold to that standard with greenbacks, as well, until my cab exceeded the wattage of the amp. My preference. G' Luck!
My Father has that amp.. Seems like he always wants to play one of mine. Not bustin'. But I've played it through my speakers, and it didn't seem to work for either one of us. Rhythm channel has some service-able tones, though, and he rather enjoys the line out to his PA.. I'd save 'em for something else, or otherwise present another 50W load, to protect your investment. This may not be the right advice, but it is the safest, until you get to the near perfect tone you're looking for, and your willing to risk it all for the next inch.
