Boston has a music legacy that needs no introduction. Think of Aerosmith, the J. Geils Band, Morphine, and Willie Alexander for starters. At the heart of the scene, though, is an artist like Dennis Brennan. He's a cagey veteran who hasn't won similar fame but can electrify audiences with his ability to cross brilliantly from rock to soul, blues, country, and jazz, while breathing fresh life into each. On several nights a week, Brennan bops around New England clubs with a passion that is all too rare in today's homogenized music climate.