It's not that tubes play louder. They just present the midrange more upfront. 90DB through a tube amp and 90DB through a good solid state(everything else being the same) will sound totally different. In a SS system, the bass will be more fleshed out so, bass will move more air than mids so, it moves the SPL level up earlier and quicker than a tube amp will. The tube amp will be moving the air with more mids and less bass which in turn sounds louder cause it more midrange and less bass. I hope you get what I mean! Get a SPL meter and two amps( 1 ss,1 tube) and test it.
