New York's top rehearsal studios have been booked solid all week as frantic preparations for the massive Saturday night birthday party for Atlantic records are completed. "It's one big family reunion," said a Ken Ehrlich, who's producing the event's HBO simulcast. As he spoke the Stax rhythm section of Booker T Jones on organ, Steve Cropper on guitar and Donald "Duck" Dunn on bass pumped out the groove on "Everybody Needs Somebody to Love," with Sam Moore belting out lead vocals. LaVern Baker, the legendary singer from Atlantic's earliest days who would soon take the soundstage to rehearse her classics "Tweedle Dee" and "Jim Dandy," watched carefully. "She hasn't been in the country for 18 years," said Ehrlich. Elsewhere around town, Led Zeppelin, the Vanilla Fudge and Iron Butterfly were rehearsing their reunion sets. Thursday night all the participants took time out to attend a pre-concert bash at Roseland to celebrate Atlantic's 40th anniversary. Partygoers danced all night to the music of San Francisco parodists S.T.U.P.E.D. (the society to undertake the preservation of endangered dumb songs) and New Orleans great Buckwheat Zydeco.