Stereomour, which is the same basic circuit and operating points as Paramour, runs the 2A3 at its maximum dissipation and maximum voltage. The normal biplates are quite tough, and of course the modern monoplates are mini-300Bs and most of them can handle quite a bit more power and/or voltage - at one point the Sovteks were said to be good for 23 watts dissipation.
I too have heard intimations that the original monoplate is a little fragile. I would not operate them in Stereomour if they were mine, out of caution for a rare and valuable tube. But I have no direct experience and no actual data; I don't think I've ever heard exactly what the problem is.
I can speculate that the originals were either difficult to make, or had a high rejection rate, or a high failure rate in service. There must have been some reason they were quickly replaced, after all.
One could probably operate them as if they were 45s, reducing both the plate voltage and current - but as I said, I don't know what the risks are, so I can't make any actual recommendation.
