Is it the fact that it's made out of solid (rather than ply) wood, but then not all "solid" basses are necessarily "professional" grade too?
Is it the fact that it's made of woods with fancier grain patterns and higher quality parts, but then many recent Chinese and Romanian basses already have all these yet they are still considered "student" grade?
Is it the fact that it's a pedigreed instrument, so that the suggestion is that the legendary Italian, French, German and English luthiers made no "student" grade instruments?
Or is it the fact that it's been played by a professional bassist, so that a plywood instrument could potentially be considered "professional" grade too?
V.
