Finally, the main reason people like tube amps is because of their "special" characteristics. They sing in a way that few solid-state amps can match (harmonics, baby, harmonics!). Because of that, we simply don't mind having less power.
Yes, and too much capacity (watts) reduces the even-order harmonic output.
Hence, a 150 watt tube amp may actually sound much less "sweet" than a 17 wpc tube amp.
Interesting story. A government contractor developed an extremely flat audio amp for telecommunications.
Perfect reproduction fron 10 Hz to 30 kHz. No phase distortion, THD nearly zero.
They were very pleased with their creation and anticipated a huge side benefit in commercial sales to audiophiles.
To cut to the chase, no reseller would buy it. It sounded too "flat", too "brittle", not bassy enough, lacking image, blah, blah, blah....
Coloration in sound is where it's at.
Everyone seems to want a "musical" amplifier.
BTW, transistors, and especially op-amps are faster than tubes.
The lack of a transformer helps this along.
The inter-electode capacitances, thermionic noise, and a host of other "bad" tube characteristics seem to be what makes them so sonically appealing.
IMHO, to simulate the sound, roll-off the top end, add a very, very small amount of pink noise, and add 2nd order harmonics to taste.
Also, do NOT confuse speaker efficiency and sensitivity.
These are two distinct charateristics.