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

Tubes are transformer coupled and have more power on a wpc basis 

tubes are voltage based ,Solid state is current driver , better image and for sure more defined Bass ,if both are quality products,, that too is why Loudspeaker efficiency and  matching is critical .

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.

How many tube amps have a damping factor of even 10 into 8 ohms, especially with 4 ohm speakers(halving the damping factor) that are so common today? I can only think off the top of my head of one tube amp with a high damping factor, the old Melos amps that had a damping factor of 20(unpublished Atkinson test). Every published Atkinson test of a tube amp I've seen(and I've been a subscriber since before Atkinson) shows substantial frequency variance with frequency.

@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.