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

@mapman suggested a spl session, which I second.  You phones ought to have one as a matter of course imho....  Quit guessing how loud is loud.... ;)

Having grown up ( years-wise, anyway ) with tubes, I kinda developed a recalcitrance to rolling and trolling for ones that would keep the TV up, later with early 'audio' about.

For sure, the early ss stuff had issues....it's typ with anything new, like D.....
D amplification has improved rather quickly, a lot faster than the early ss did.

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.  

Add in the noting of impedance variables, the different characters' of pre's, amp's. cables, ic's. source types....

...and the room you're in and the chair under you.

All that said, if I stumble upon a tube amp I recognize from y'all that can be stolen for a pence or three, consider it bagged.  I'm curious about how my Walsh will react to a different sort of feed.... ;) *G*

For the way I listen, a crucial part of any audition is how softly the gear can be played and still sound full and engaging.  How loud it can go is largely irrelevant if it gets loud enough.  It’s like the old horsepower rating on Rolls Royce—“sufficient.”

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 .