OK. I am taking about amps that are typically made in a box and played into a closed speaker cab.
About a decade ago when I had lots of disposable cash, I was doing alot of recording with some big 6L6 and EL34 100 watt amps, and I also happened to own a 50 Watt 6L6.
When you crank the amp to the point it starts to saturate in a pleasing way, the 100 watt amps still have punch and definition. That have this very alive and screaming sound. They have more headroom.
The smaller amp saturates sooner and doesn't have as much headroom. If you are playing in a metal band with a loud drummer, and have lots of clean passages where you don't want any distortion, the 50Watt is not ideal. And it doesn't have the same punch either so it can disappear more easily. This wouldn't matter in the scenarios where open back cabs are typically used.
I'm not a recording engineer or a professional, but I did alot of experimenting, listening in isolation to miced speakers at various volumes, and there is a reason 100watts is the standard for tube heads. It is the right balance of saturation and punch. I've also owned EL84 amps and I have never preferred them. You can even hear on youtube demos that EL84 lunchboxes don't have the same magic as their larger counterparts.
Its the kinda thing that unless you have experienced it yourself, you probably won't know what I'm talking about. I get it.