Output watts per tube

I originally posted this question in the amp/pre-amp section with limited response. Thought I'd try it here, with some corrections. I'll further preface this by saying that I am not a technical guy, so everything I'm telling you here I learned last Thursday - fair warning if there are any gross inconsistencies with my explanation.

I recently visited a guy locally who was able to test the dozen EL34 Ruby output tubes from my Cary V12, with interesting results. Despite having 6-800 hours on them, ten of the twelve tested between 100 and 110 on a tube tester, and the other two came out between 85 and 100. This was using the sensitivity set at 53, per that tester's standard for the tube type.

Then, we used a sine wave scope with three tubes to get an average control measure, then checked all of the tubes against the control for distortion (none) by running two tubes in a test amp running push-pull in class A. This was also hooked up to a meter to measure output watts. (someone asked, and I don't know what the plate voltage was used in the amp) The result was that those two tubes - any combination of two of the twelve - had a sustained maximum output wattage of 21 watts, even the two that were a little low on the tester. The results from tube to tube were, unexpectedly, almost perfectly linear. Given the Rubies less than stellar reputation, I was expecting some measurable degradation or at least inconsistency.

So here's my questions: Is the 21 watts for two tubes the equivalent of 10.5 output watts per tube; and if my amp takes twelve tubes and makes 50 watts per side in triode, am I in fact only using a little over 8 watts per tube at any given time? The amp can also run at 100 watts per side in ultra-linear, which would require closer to 17 watts per tube (six per channel). Does the 50 Watts equate to relatively low stress on the tubes and longer tube life? Does employing 12 tubes somehow reduce the per-tube output further? (Cary didn't have an answer for any of this)

Any thoughts?