Musical innovation is a slippery, ill-defined concept. Does it live in novel melodies, or mash-ups of styles? Is it something made by fingers on frets or in the minds of the audience? I suppose it falls in the I-know-it-when-I-hear-it category. Or I could just point you to a Cadillac Sky show. Ostensibly a “bluegrass” band, the five C-Sky guys are an ever-changing ensemble of artists who absorb top flight influences but who make sure that what comes out the other end of their creative black box is always searching and never derivative.