Backstage before the eighth night of the Allman Brothers Band's recent 15-night run of shows at New York's Beacon Theatre, 61-year-old drummer Butch Trucks can barely contain his excitement about tonight's secret guest. "We've been trying to do this for 40 years!" he says, referring to the first time the Allmans will share a stage with Eric Clapton. "He's been borrowing our guitar players since the Layla record," says Gregg Allman with a chuckle. "So he kinda owed us."

For two nights, Clapton joined the band to tear through tunes including "Anyday," "Why Does Love Got to Be So Sad?" "Little Wing" and, of course, "Layla." His unannounced (but widely rumored) appearance helped the Allmans celebrate their 40th anniversary — and honor co-founder and guitarist Duane Allman, who died in a motorcycle crash in 1971. "To be honest, one of the reasons we're doing this is that Duane doesn't loom big enough," says Trucks. "Too many people don't even know who he is."

During the 15 nights, an astonishing list of old friends joined the Allmans. "We tried to get everybody that my brother had played with," says Gregg. Boz Scaggs played his 1969 classic "Loan Me a Dime," which originally featured one of Duane's greatest solos; Levon Helm sang "The Weight" and "Ophelia," and Taj Mahal brought the house down with a lead vocal on "Statesboro Blues." After initially rejecting the idea of inviting guitarist Dickey Betts, who was fired from the band in 2000, the Allmans sent him an invitation; at press time, Betts' participation was still undetermined. "If he does come, it's not gonna be easy," says Trucks. "There's a lot of bad blood. But for the sake of Duane, we decided we would ask."

Most nights began with a photo montage of Duane and a rendition of "Little Martha," from the classic 1972 disc Eat a Peach, centered around current guitarists Warren Haynes and Derek Trucks. The Allmans also made sure to invite younger musicians who are breathing life into jam-band traditions, including Phish's Trey Anastasio and Page McConnell, who elevated "Southbound" and "In Memory of Elizabeth Reed." "I've been playing Allman Brothers songs more years than I care to count," says McConnell. "After being in a band a number of years, you respect more and more the longevity and stamina it takes to do what they do year after year."