By Paul Robicheau

Umphrey’s McGee’s UMBowl X was an exhaustive two-night series for non-diehards but rollercoaster nirvana for the initiated.

Umphrey’s McGee, at House of Blues, UMBowl X. Photo: Paul Robicheau

Jam bands have a thing about springing conceptual tricks in concert. Phish set the standard, mounting Broadway-level productions on New Year’s Eve and recently taking visuals to new heights at Las Vegas’s dazzling Sphere. Other bands from moe. to Goose have pulled their own holiday hi-jinks, and any night with Disco Biscuits can be wound with inverted-song puzzles.

But Umphrey’s McGee takes the idea of themes to extremes at UMBowl, its semi-annual takeover of one city that hit Boston’s House of Blues on Friday and Saturday. The two-nighter was postponed from last Halloween (a delay that may have contributed to Friday’s lighter attendance) to give crack drummer Kris Myers the time to recuperate from rotator cuff surgery.

Jake Cinninger and Kris Myers in Pan’s Labyrinth at House of Blues, the UMBowl X. Photo by Paul Robicheau

After more than two decades with the same lineup, Umphrey’s remains one of the jam scene’s most technically proficient ensembles, its brand of precise fusion spanning metal, funk, reggae, and space rock. And the sextet tapped all those styles during UMBowl X’s six hour-ish sets of mental and physical gymnastics through themed improvisations, old and new originals, covers, and mashups.

Not every set clicked, though the high points proved both novel and engaging. Fans had a hand in some of the triumphs and failures through advance and real-time voting. Friday’s first set began on solid footing with a “Band’s choice” of originals, culminating in a 35-minute sandwich of “Bridgeless” around “The Triple Wide.” Guitarists Brendan Bayliss and Jake Cinninger wove counter-trills in harmony into an escalating jag, screen images rotated like a kaleidoscope, and the musicians settled on the other side with patient muted-note runs.

Brendan Bayliss at House of Blues, UMBowl X Photo: Paul Robicheau

However, Friday’s second set — a “Stew Art” event where fans contributed real-time improv themes — went steadily downhill. It began promisingly with a suggestion of “Drums and brace yourself” that led Myers to lash into cycling breakbeats that anchored a whiplash guitar swirl. And “Sludge and dub” took a literal move from grungy reverb to choppy reggae. But ideas for Cinninger to conduct the others, for the lights to drive the band, to play on one string (there was some cheating), and for everyone to play keyboards (what a band practice might sound like if everybody dropped acid) and then the drums went nowhere interesting fast. The degradation was complete when “Play your favorite song” led Bayliss to sing a solo acoustic “Puff the Magic Dragon” in which he twisted the lyrics into impish drug references, ending with bongs and sexual toys.

Luckily, Friday smartly ended with an old standby, fan-voted covers, launched by Cinninger’s deft harmonic intro to “Roundabout” on electric guitar. “Kid Charlemagne” was even better. Bayliss had Donald Fagen’s vocal phrasing down pat and took the bulk of its guitar solo with carefully fingered crispness, while Joel Cummins lent the requisite electric piano. And Umphrey’s crushed Stone Temple Pilots’ “Plush,” Living Colour’s “Cult of Personality” (Cinninger injecting speed clusters on guitar while Myers nailed its gear-shifting power drums), and Temple of the Dog’s “Hunger Strike,” where the drummer doubled up, soaring through Chris Cornell’s high vocal parts. Percussionist Andy Farag stepped out to rap with Bayliss on The Notorious B.I.G.’s “Big Poppa” for a final touch of levity. Even if the band didn’t hit every change perfectly in an encore of the tricky Rush warhorse “Tom Sawyer,” its jukebox skills saved the night.

Saturday gave a near-full House of Blues a more consistent, creative ride, starting strong with fully improvised scores to fan-voted movie clips. After some dark, droning play on effects pedals to classic horror films, Umphrey’s hit full throttle as it accompanied a Mad Max: Fury Road chase. Myers kicked in as postapocalyptic mutants pounded drums on the back of a battle vehicle, and the band synced to chaotic action with a gnashing avant-metal whammy. The dance scene from Pulp Fiction lent a twangy changeup before a Blues Brothers chase clip went on far too long but Pan’s Labyrinth built back fantasy tension. Likewise, footage from Interstellar, Caddyshack (with scratchy, playful funk to a teen pool takeover), and a Kill Bill sword fight entertained the band and fans, though they never reached the apex of Fury Road.

The middle set supplied a mixed bag of never-played material. A new cover came first in a faithful stretch through Dire Straits’ “Sultans of Swing,” then wound through “Concessions” (a patchwork assembled from past live “Jimmy Stewart” improvisations) and “Work Sauce.” But the high point, along the lines of Umphrey’s 2014 Halloween show at the same venue, was a new mashup. House of Pain’s “Jump Around” (with the rapping Farag cuing fans to jump in place) morphed into Blondie’s “Heart of Glass” and back, while adding touches of Tame Impala’s “The Less I Know the Better” to the familiar surprises.

Umphrey’s McGee at House of Blues, UMBowl X. Photo: Paul Robicheau

Again, Umphrey’s knew how to close out UMBowl X with a winning final period in “Choose your own adventure.” Fans texted, via an instant poll, to pick between two to three old favorites that flashed on the screen. They were well-trod originals floated by the band, so Umphrey’s couldn’t lose. Some tunes won by a landslide, including the disco-y vamp “Night Nurse” and sinewy, break-laced “Plunger.” Others were tight contests, including a funky “Bad Friday” (which only won via a voice vote over “Miami Virtue”) and “Making Flippy Floppy,” edging out its Talking Heads alternate “Girlfriend Is Better.”

Either way, fans suggested that they wanted to dance in the final frame while also basking in triumphal shots of “Glory” (where Cinninger let loose with high-sustain leads) followed by the theme from Top Gun accompanied by footage from the film. The latter was at the center of  the encore after Bayliss returned to add a verse to “Puff the Magic Dragon,” only for bassist Ryan Stasik (sporting a Larry Bird Celtics jersey) to smash the acoustic guitar.