"tube watts" versus "solid state watts"

I'm sure you, like me, have seen it written more than once that brand X tube amplifier, rated at 30WPC, sounded more powerful / more authoritative than brand Y solid state amplifier, also rated at 30WPC. Or that brand Z tube amp, only rated at 15W, was comfortably able to drive brand A speakers, because those 15 were tube watts and therefore up to the job. Heck, I think I've even heard the phenomenon with my own ears.

My question is: is there any basis in electrical engineering for this effect? Can we say scientifically what's going on here?