Courtesy Jay Blakesberg

The Waybacks — (from left) Joe Kyle Jr., James Nash, Chuck Hamilton and Warren Hood — expect to do a lot of jamming with the John Cowan Band at the Kirk Avenue Music Hall show.

Courtesy John Cowan

John Cowan rose to fame as the lead singer for New Grass Revival.

Americana/roots act The Waybacks and newgrass pioneer John Cowan put on a highlight show for hundreds at last April's edition of MerleFest.

A thick crowd gathered at the North Carolina music festival's Hillside stage for what is becoming an annual must-see: The "Hillside Album Hour," with Cowan, The Waybacks and some high-caliber musical guests.

Roots music fans who missed that gig can catch up with both acts when they hit Kirk Avenue Music Hall on Wednesday. It's a co-bill for Cowan's band and The Waybacks. Count on some interband jamming, Waybacks leader James Nash said.

"I think there's going to be no way to get us out of there without it," Nash said in a phone conversation last week from his home in San Francisco. This and a couple of other dates with Cowan's group are "just a very thinly veiled excuse for us to get to jam at the end of the night together. I'm not even going to tease people and say it might happen. It absolutely will happen."

Each act will play about an hour set. "Then we're going to get together and jam it out," he said.

A classic jam

The "Hillside Album Hour" started in 2008. Nash chose the record "Led Zeppelin II" that year particularly for Cowan, figuring that aside from former Zep frontman Robert Plant, Cowan was the only singer on the planet who could pull it off. He was right, and an institution was born.

By the time The Waybacks, Cowan and former New Grass Revival bandmate Sam Bush hit the stage that Saturday afternoon with world-class resonator guitarist Rob Ickes, a guessing contest about the featured album had already developed.

Nash had previously dropped some difficult clues online for the new faithful, and a few had already decided that the hour was going to be about The Rolling Stones' "Sticky Fingers." Those folks were right.

The crew opened with "Brown Sugar," and many in the rest of the crowd recognized it immediately. But there was an unexpected surprise coming. Country music goddess Emmylou Harris took the stage to sing lead on "Wild Horses." Harris had been a protege of the late Gram Parsons, a Rolling Stones friend, and had sung backup on Parsons' version of the song.

The crowd was wild about it. But maybe no one was as amped about it as Nash.

"With Emmylou, I didn't know if she was going to be doing it until literally the day before," Nash said. "I'd invited her to do that a few weeks [before]. I just thought, how amazing it would be to have her sing 'Wild Horses,' you know, especially with her connection with Gram Parsons. And I was a little bit apprehensive about it, because I just didn't know how she was going to take it. ... She's just been a hero of mine all my life, so I was about as nervous as I've ever been meeting somebody when I got to walk into her rehearsal she was having at MerleFest."

Nash discovered that she was a "100 percent nice person -- no star affectation."

Harris told Nash she loved the song, which she thought was one of the greatest ballads ever written. She wanted to do what was best for the song.

"I just thought that was so sweet and nice from somebody who could've just as easily said, 'Who the hell are you? Leave me alone,' " Nash said. "And of course, the way she sang it ... it was haunting. ... I think everybody up there on stage had chills, like I did."

That hour might be tough to top. But Nash said that he and Cowan have recently discussed next year's possibilities.

"We want to do it next year, and I think it's going to work out," he said. "And we may have picked it, but I'm not going to tell you [what it is]. ... What I will say about it is, if we go with the album that I think that we're going to go to, it will be the most difficult one that we've done so far."