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

Tubes can have more headroom, softer clipping, and their harmonic distortion adds to the perception of loudness. And they’re just more awesome.

Perhaps it is simply more sensitive at input voltage and just sounds louder?

Maybe because the particular tube amp you are enjoying has a great transformer....;0)

It's not really about loudness, it just feels more powerful at the same volume.

But I would say that in my case it is an excellent amp/speakers electrical match, the amp drives them with effortless grace at any sane level.