Yep, different people have different opinions on this, and that’s a good thing. Two ratings I have never had a lot of confidence in, waistband measurements on trousers and wattage ratings on amps.
A few times I have used a 50 watt Marshall and I have turned it all the way up to 10 in a medium sized room, no problem, they don’t seem that loud to me. I find a 100 watt Marshall to be roughly 2.5 times as loud as a 50 watt, but everyone else tells me they are only 1.5 times as loud. You’re absolutely right other lower wattage amps seem louder than their rating suggests.
I love the sound of power tubes maxed out. It’s a lovely smooth, sustain for days that you can feel in your guts.
I don’t think that power amp distortion or even tubes are essential for the modern metal sound, so there are ways of getting that tone without 6L6s as an essential part of the equation. A lot of the sound is in the tuning and strings on the guitar, tune it down, get some nice tight response overdrive happening boost the bass and the mod metal tone should happen.