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

In my experience, tubes tend to highlight the lower part of the spectrum, where s/s favors the upper. 

Tube amps also tend to boost the mid bass and mid range (from about 1 kHz to 2 kHz or ao) due to their low damping factor interacting with speaker impedance.

@dynamiclinearity This is only true if the amp is unable to act as a Voltage source on the speaker in question. Many tube amps are perfectly capable of such a thing. You only need about 15dB of feedback to achieve that in many cases.

In my experience, tubes tend to highlight the lower part of the spectrum, where s/s favors the upper.

@stringreen That is on account of how the amps in question make distortion. Once you understand that, its then possible to design a solid state amp (including class D) that doesn’t sound bright.

@dynamiclinearity I'm going with 'a fair amount'.

The venerable Dyanco ST70 has a damping factor of about 15. This does not change with a 4 Ohm load since you'd be using the 4 Ohm taps. It would be cut in half if you ran a 4 Ohm load on the 8 Ohm tap.

ElectroVoice made a number of tube amps (such as the A30) that had a damping factor of 15 back in the 1950s. It was EV and MacIntosh who came up with the idea of the amplifier being a Voltage source and the speakers being Voltage driven; this to enhance plug and play.

Prior to that speakers had adjustments on the back (midrange and tweeter level controls) to allow the speaker to be adjusted to the unknown output impedance of the amp.

The Mac MC275 had about 15:1. The Marantz 8b amplifier had a damping factor of 16:1.  The famous Fisher SA100 was 20:1.

FWIW no speaker made needs a damping factor of over 20.

Then why do almost all the damping factors measured by Streophile fall below 10 and usually well below. And mor observationally why isn't the frequency response of amps shown by Stereophile into a simulated speaker load so far from flat?