Ralph, in the interest of optimizing sonics do you think it would be worthwhile for Robert to try a pair of 5751's in the 12AX7 locations?
It might be worth a shot- of course an easier tube to find is the 12AT7, which has the same mu: 70 instead of 100 of the 12AX7.
I prefer both tubes over the 12AX7 as its a lot easier to get bandwidth. I would think that you want as much gain as possible without pushing the amp into instability. The problem here seems to be that the feedback loop does not have proper cutoff frequencies set up, so with a heavily capacitive load (and in UL operation, which will have a little more voltage output due to the increased power and reduced grid capacitance) it can become unstable. Sorry for the technical mumbo-jumbo.
Apparently the condition is not able to damage the amp over short duration, so I think its worthwhile to try the various tubes (12AU7, 12AT7 and 5751) to see which offers the best sound along with stability. The only trick is that depending on the tube brand, the sound might vary enough that the brand may supersede the type. That is good if that's the case, then its all just about the best sonic performance.