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

i tend to find, particularly with lower powered tube amps, the sound tends to be more full and rich sounding and more lively at any given volume level than is the case with solid state.  Solid state tends to sound a bit dead and less engaging which prompts one to turn the volume up more.  That one tends to be satisfied at lower volume levels with tubes, I suspect, makes one feel that tubes are delivering more power than they are actually delivering.  Also, most people don't come close to actually using up the massive "reserve" of power from their amplifiers.  So, something that sounds rich and lively at low power is not disadvantaged as much vis-a-vis much more powerful solid state amps; the advantage of extra power in reserve tends to be more theoretical than practical. 

As long as the tube amp is working at or below about 1/3 or 1/4 of its rated output it will sound sweet and uncompressed.  While that power level might see, really low, it is probably a limitation that is breached for only short periods for most listeners not requiring head-banging levels.

Nelson Pass wrote an interesting paper on this and I think this is it.

It doesn't directly say tube amps but rather amplifiers with high impedance.  Worth a read. 

Contrary to popular opinion, I greatly appreciate how much Nelson Pass does for the audio community, and this is an example of why.

All of the above true, and the biggie for me is tube's special way of delivering micro dynamics, Brits call them valves which brings to mind human lungs and breath, tubes give breath to performers.

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

@inna, Mapman has it right. The reason the tube amps sound like they have more power is because of how they make distortion when approaching full power. Unless its an SET, with most tube amps the higher ordered harmonics start showing up in greater amounts near overload. This interacts with the ear which imparts the impression that its louder- more powerful. This has given rise to the false notion that tube power is more powerful than solid state.

But like @mapman pointed out, if you have a sound pressure level meter, you'll find that its not louder- and in fact might be quieter.

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).

Tubes make it an enjoyable experience since there's plenty of second order harmonics to go around too.

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.