When Gov't Mule paired with John Scofield during Sept. 1999 for a pair of shows in Georgia, Warren Haynes acknowledges it "was definitely a head-scratcher" for the Mule's then-burgeoning fan base.

"I think there were some people in the audience that had no idea we were going to do that much instrumental music and that much jazz-influenced music," Haynes tells Billboard.

"But it still sounds like us. When I hear it now, it sounds just as fresh as it did then. It captures us meeting for the first time, musically speaking. It started out as an experiment and turned into something that kind of changed the course of our music."

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Haynes and the Mule are letting others hear that music -- officially, at least, after prolific bootlegging during the interim -- with Sco-Mule, a two-disc set coming Jan. 27 as part of a series of archival releases to celebrate the band's 20th anniversary. Billboard is excited to exclusively premiere "Hottentot," a Scofield original from Sco-Mule, here:

"I think our intent was to release it about a year later," says Haynes, and indeed Sco-Mule's first disc was even mixed for release back then. But after the August 2000 death of bassist Allen Woody, "everything just changed" according to Haynes. "We didn't feel like it was the right thing to put these recordings out at that time. In the beginning we didn't even know if we were going to keep going or not. But it's been on our minds for a long time, and in the long run, I think it's probably a good thing, because I don't know that the Mule audience would've totally been ready for it. I think they're much more ready for it now, and now we can do a tour with John to promote it. It all kind of makes sense and it ties into the 20th anniversary, so I guess it worked out."

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For his part, Scofield vividly recalls, "Man, Mule plays loud as hell!" His fans scratched their heads and raised their eyebrows over the collaboration, too, but the guitarist felt a kinship with the "Southern men" of the band.

"I started off with blues and rock as a kid, pretty much like everybody who plays the guitar," says Scofield, who played on Mule's The Deep End, Vol. 2 album in 2002 and went on to work with Haynes in Phil Lesh & Friends. "A lot of jazz guitarists shelve that aspect and get rid of their rock n' roll, but I never did. So when I went down and played with Warren and those guys, I kinda knew what was going on. And at the same time Warren and Mule have always had an affinity for jazz and wanted to bring the jazz element into their music, just like the Allman Brothers and the [Grateful] Dead did. It was a good match."

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Sco-Mule combines both Mule and Scofield originals, plus expansive renditions of Wayne Shorter's "Tom Thumb," the JB's "Doing It To Death" and "Pass the Peas," the Allman Brothers' "Kind of Bird," and a titanic romp through Mongo Santamaria's John Coltrane-popularized "Afro Blue."

Scofield says Haynes plans to ready all the Sco-Mule tracks for the upcoming tour, which begins Feb. 18 in Seattle and includes 18 shows into March. "He just sent me a list of 50 tunes -- that's five-zero -- that he wants to play in addition to that, a bunch of tunes I don't know. He wants to really be playing different songs every night. That's gonna be a real challenge, and fun."

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Haynes says the new shows with Scofield will be recorded with a Sco-Mule 2 in mind. "Hopefully it won't take 15 years for that to come out," Haynes quips. Mule's next, and final, 20th anniversary archival release will be Dub Side of the Mule in April, taken from a New Year's Eve 2006 show that featured an hour-long jam with Toots Hibbert and other guests such as Gregg Allman and Blues Traveler's John Popper. That follows last year's releases of Stoned Side of the Mule: Volume 1 and Dark Side of the Mule, which featured the group's version of Rolling Stones and Pink Floyd favorites, respectively.

"Those are head-scratchers, too, but it's our 20th anniversary. We're doing a lot of crazy shit," Haynes says with a laugh. "One of the things I really do love about these four archival releases is if you look at them as one overall picture, very few songs are repeated. So if you're a hardcore fan and you want all of it, it's a lot of music."

The end of the Allmans last year doesn't mean the Mule is the characteristically busy Haynes' only endeavor, however. He's working on an acoustic-flavored solo album with members of Railroad Earth, and he says he and Lesh are discussing some projects as well. Haynes and Mule multi-instrumentalist Danny Louis also appear on the upcoming album by POA (Planet of the Abts), the side project of Mule drummer Matt Abts and bassist Jorgen Carlsson. After those are done Haynes says he'll "look at another Mule record" to follow up 2013's South!