"I AGAINST I." Bad Brains. SST. More than a few punk bands have tried to expand the limits of hard-core thrash by varying the tempos and adopting elements of other rock styles. These excursions out of the insular hard-core womb usually leave the listener yearning for the good old days, but Bad Brains' latest is a marvelous exception. "I Against I" does feature several magnetic slam-pit numbers, but the real trailblazing tunes are a series of metallic, mid-tempo psychodramas that combine Dr. Know's chunky, lethal guitar work with H. R.'s doom-gloom vocals. The world's first Rastafarian punk outfit (live at Fender's on Saturday and the Roxy next Sunday) also throws some choice reggae licks into its potent potpourri. The Washington, D.C., quartet is a rare reminder that punk doesn't always have to be played at breakneck speed to make its mark.