Mountain Heart came on the scene with its unique bluegrass sound in the late 1990s, and that sound has endured intact through a series of personnel changes. Despite the band’s considerable success, nobody else has been able to imitate it.

Bluegrass is a quiet, often subtle, music as a rule, but Mountain Heart is all about power. The six members of the group make a big noise, and when they do a long traditional jam like “Lee Highway Blues,” they can turbocharge it like nobody else in the business. But most of their music isn’t traditional. Alone among progressive bands, they are most influenced by the sound of mainstream commercial country and Southern rock. Many of their concerts even include an extended version of the Allman Brothers’ “Whipping Post,” sung and performed on piano by multi-instrumentalist Josh Shilling.

That makes them something of an odd fit for the Ark, downtown’s venerable folk club, where the bluegrass tends to run toward the deep Monroe-based music of the Del McCoury Band and, more recently, the jam band influence that started with the Grateful Dead. But fit they do. Mountain Heart has come to town during the slow phases of early January for the last several years and has just about sold out the Ark every time. The synergy between band and audience is obvious to anyone in attendance, and the rafters ring.

Something is happening in addition to the band’s virtuoso abilities, Shilling’s full-throated vocals, and even beyond the remarkable abilities of banjoist and founder Barry Abernathy, who was born with only a thumb and an index finger stub on his left hand. That something involves the intersection of music, technology, and space. Bluegrass bands rarely record live albums, because at the low-tech venues where they tend to appear they’re lucky to get even adequate sound. But a Mountain Heart show is different: it’s a sophisticated audio enterprise, involving such devices as in-ear monitors, that takes time and expertise to set up. The band recorded its live album, Road That Never Ends, at the Ark in 2007, and you get the feeling that they do their best work at the club because they know the show is being done right.

Mountain Heart returns to the Ark on Friday, January 13.