Why do tube amps often subjectively sound more powerful than SS ?

It’s the low-order harmonic distortion which makes tube amps sound "full" and satisfying at the same SPL levels. Never sterile, lean, or bright. It’s also probably the fact that their front-ends give a lot of gain (6SN7, 12AT7, 12AX7 etc), so often lower powered tube amps will actually give you more SPL output at the same preamp / control volume setting, versus solid state amps with even higher power ratings (@mikhailark pointed this out via senstitivity). In these cases, you have to raise the volume dial to tap into a SS amp’s power reserves - past the point that would be safe for the tube amp.

Gain is NOT power - when it comes to the limits, power is power and you will hit clipping all the same, at the amps’ actual limits of power, whether SS or tube. Tubes might be polite and give you a "warning" first - compression and then a soft-crunchy garbled distortion before the hard, nasty clipping.

I mentioned the caveat of SETs- they take this to an extreme by producing those higher ordered harmonics at a much lower power level- perhaps only 20-25% of full power, which is why many people talk about their 'dynamics' (since power is used the most on transients). 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).

You've mentioned this before, and I finally got to experience & hear this phenomenon for myself. Spot on. Push-pull amps are better for me, especially since I like to listen loud. 

So when I got the Bryston amplifier, I thought, since it was more powerful, put on really powerful music that will take advantage of it like the Conan soundtrack.

 

I put in the Bryston, cranked it up and…huh? Where was that power?

Why did it actually sound thinner?

Less impactful?

Man, I love that soundtrack. And film.
Yeah, another way I can put my experience with various amps - push pull tube amps tend to sound "good to the last drop". I listen loud and I’ve pushed hard on some vintage PP tube amps in the 20 - 35 Watts / ch range. Until you hit hard clipping, they sound amazing (euphonic) all the way through. Sweet, fat tone.

With solid-state there’s more of a phenomenon like: when you crank it up, you start hearing stuff presented in a way that makes you want to back down the volume again. This can occur even when theres’ still AMPLE power reserves left before approaching hard clipping limits. With tubes, you may keep wanting to crank it up until you actually hit hard clipping.

Probably due to a combination various factors - like harmonic distortion spectrum (which Ralph has educated many of us on), and possibly some dynamic compression with tubes (as another poster mentioned) that can actually work toward the presentation’s favor in some instances. "Loudness wars" with regards to digital mastering gave dynamic compression a bad name, but it’s not all bad.