For the better part of the last 25 years–ever since he joined Herbie Hancock’s bands in the early 70s, having already spent a decade playing with such California stalwarts as Bobby Hutcherson and the Crusaders–Buster Williams has practically defined the role of the modern jazz bass. After Charles Mingus and then Scott LaFaro began to free the instrument from its traditional timekeeping function, bassists like Eddie Gomez and Stanley Clarke headed off in another direction, turning the instrument essentially into another front-line melodist rather than a supporting fundament of the jazz combo. It remained for Ron Carter, and then Cecil McBee and Buster Williams, to reunify these separate streams into the current model for jazz bass: expressive and independent, but still a deep-toned anchor for the entire ensemble. Williams displays his mammoth technique without exhibitionism, and his virtuosity proves all the more impressive for its matter-of-fact quality. He produces a tone best described in wine-list terminology–dry, velvety, with tremendous body–which has enriched the literally hundreds of albums on which he has appeared. (Such qualities have not gone unnoticed among the most demanding critics–namely, other bassists. When in the mid-80s Ron Carter formed a band to feature his own solo work on the higher-pitched piccolo bass, he hired Williams to play the traditional bass-fiddle part.) Williams’s expansive solos have bite and proportion, but you could listen to whole concerts of his immaculate and imaginative accompaniment and still leave with no complaints. For this visit he leads a trio featuring a pianist named Carlos McKinney and the voluble and voluminous drummer Aaron Scott, who has spent the last seven years stoking the fulminant fires of McCoy Tyner’s music. Friday, 11:30 PM, and Saturday, 9:30 and 11:30 PM, Bop Shop, 1807 W. Division; 235-3232.