Well I seem to be getting mixed reviews here. I will say that there is only 3 decibels difference between the 50 and 100, but i played them both (soldano avengers 5881) at noon, as that was right before power saturation started having the mush effect. The 100 watt had absolute punch. And no need for the depth knob whatsoever. The 50 was just thinner, and quieter. 3 decibels matters to me, I guess. Both of them at that volume, I would keep the preamp gain at 11'oclockish because you don't really need preamp gain to be unruly at a high volume when an amp is breathing fire.
I have a 100 watt el34 amp now. and i can't make myself take it near the noon area, or haven't tried, because i am at 105-110 decibels depending on how hard i hit the strings at a little over 9:30-10 on the master . Power amp starts-i don't know if saturating is the right word here-getting involved pretty heavily in a bell-like quality way (best way to describe it) around 10:30 to 11.
Basically based on my experience, which to be fair never included a stock high gain el34 50 watt amp, I have not been impressed with the lower watt amps. Some of you have said that the 50 watt will play a stadium or tear your head off. That is fine, but will it still stand up next to the 100 watt version? That's kinda what I wonder.