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

Perhaps it is simply more sensitive at input voltage and just sounds louder?

Maybe because the particular tube amp you are enjoying has a great transformer....;0)

It's not really about loudness, it just feels more powerful at the same volume.

But I would say that in my case it is an excellent amp/speakers electrical match, the amp drives them with effortless grace at any sane level.

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.

@inna - how do you calibrate "same volume"? Literally 1% of sensitivity difference makes it well audible. And yes, tube distortions add to the sound.