@mapman suggested a spl session, which I second. You phones ought to have one as a matter of course imho.... Quit guessing how loud is loud.... ;)
Having grown up ( years-wise, anyway ) with tubes, I kinda developed a recalcitrance to rolling and trolling for ones that would keep the TV up, later with early 'audio' about.
For sure, the early ss stuff had issues....it's typ with anything new, like D.....
D amplification has improved rather quickly, a lot faster than the early ss did.
But what they are really talking about and also in this case is distortion and how it interacts with the human ear (the ear uses higher ordered harmonics to gauge sound pressure.
Add in the noting of impedance variables, the different characters' of pre's, amp's. cables, ic's. source types....
...and the room you're in and the chair under you.
All that said, if I stumble upon a tube amp I recognize from y'all that can be stolen for a pence or three, consider it bagged. I'm curious about how my Walsh will react to a different sort of feed.... ;) *G*