A few years ago a segment on Comedy Central’s The Daily Show strung together video clips of several Congressional representatives pejoratively referring to technically proficient specialists as “nerds.” Host Jon Stewart stared into the camera, and in perfect deadpan, suggested, “The word you’re looking for is experts.” Think of progressive rock’s cranial compositions as similarly, and, too often, dismissively typecast; its creators and fans, alike, having “nerd” tattooed proverbially on their musical foreheads. Then, spend an evening with the Dixie Dregs, and, yes, you can guess the word you’ll be looking for after their two-and-a-half hour, two-set brushfire of a performance.

The Dregs are back, reuniting for the first time in 40 years the five original members of this instrumental strike force, and they’re nasty good. There’s goateed bassist Andy West, equipped with a gossamer touch and a low-end blasting a cannonball to the abdomen. And guitarist Steve Morse, his long, stringy hair and the stovepipe arms of a Viking arm wrestler, tearing off fretboard runs that shuttle fluently from delicate to dangerous, lilting to light-speed. Or Allen Sloan on violin, step for step with Morse, putting bow to string with emotive taste and sniper’s precision. As well, there’s Steve Davidowski fueling the frontline blaze, his keyboards comping melodic patterns, plus solo flares of his own. Behind it all, Rod Morgenstein, the indefatigable keeper of time (straight, odd, or indefinable), repeatedly rolling the thunder, supporting every syncopated flurry, every ultra-tight turn.

To a packed-in, mostly seated, post-dinner crowd, the Dregs opened with a feverish “Divided We Stand,” sliding into the funk of “Free Fall,” then the sprightly “Holiday,” before West, serving as group spokesman, took a pause to say hello. They looked ecstatic, these five, having survived all the trends that distract an audience, as disco and punk did four decades ago. For a song, West brought out his old Alembic bass- one he no longer owns but borrowed back for the occasion- still possessing a growl that loosened some fillings. They played the favored and the favorites, dipping into the catalog as far back as their University of Miami origins. They showed-off Morgenstein on “Country House Shuffle,” burned soles on the Scotch-Irish jiggy “Moe Down,” and found their psychedelic soul on the set-closing “Odyssey.”

Sloan began the second set alone, on a poem-inspired violin soliloquy. Morse returned on acoustic guitar for “Northern Lights,” as did the rest of the Dregs for the chamber-styled “Go for Baroque.” The instrumental barrages brought cheers of awe, waves of nodding heads. It was ferocity riding shotgun with calm; the controlled bursts of “Day 444,” “Refried Funky Chicken,” then “Leprechaun Promenade,” and “Wages.” The final duo of “The Bash” paired with familiar strokes of “Cruise Control;” like a Ferris wheel of virtuosity, coming around again and again.

Guitarist Travis Larson joined Little Feat’s Paul Barrere as special guests for an encore blitz of Cream’s arrangement of Robert Johnson’s “Crossroads,” with Barrere, on vocals sans guitar, belting out the blues. A last scintillating “Bloodsucking Leeches” brought up the house lights as the quintet waved farewell.

The Dixie Dregs didn’t just make the hard sound easy. They made the hard sound hard. They reiterated how skillful, how surgically exacting they are, how few can do what they do as well as they do it, and how, sometimes, to make the music really come alive, it takes the experts.