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

Generalised distinctions between tube and solid state amplifiers are meaningless - it totally depends on the particular amps being compared.

I started with tubes growing up, than switched to SS ie Hafler pre and amps in the early eighties.  While they worked great, there was something i was missing.  Later in the eighties, I went back to tubes and have never left, well kinda.  The rig downstairs is hybrid. The preamp and the amps are IC chip input with tube output and the main system is tube input and transistor output on both the preamp and amplifiers. 

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.