If you’re following closely, you’ll find repeated references to dub reggae in reviews of both When in Vanitas . . . and Tortoise. Surface characteristics do give the two records a dub veneer: unorthodox mixing, curious sounds, phase shifting, deep-in-the-pocket grooves, denaturalized echo, sudden ruptures in pulse/background relationships, and an overall sense of studio artifice. When Tortoise performed live at HotHouse in August, they played dub during the break, confirming the Jamaican influence. And some of the engineering techniques on both discs were first attempted decades back on dub plates.

But at a fundamental level the debut CDs by two of Chicago’s premier instrumental ensembles are not dub at all. Dub reggae is deconstructionist; Tortoise and Brise-Glace are constructionist. In the late 60s, innovative studio electrician King Tubby (aka Osbourne Ruddock) began to take existing reggae recordings, strip them of their vocals, and perform open-reel surgery on them. He added echo and sound effects, rearranged the bass-treble balance, sometimes dropped the drums or bass or keyboards or guitars for a few bars, grafted little nonsensical vocal prunings back into the mix, and basically created something brand-new out of the vivisected carcass of the original song. Dub Gone Crazy collects primo material recorded from 1975 to ’79 at Tubby’s studio and mixed by Tubby, Prince Jammy, Prince Phillip, and Scientist; it was compiled by reggae’s most expert curator, Steve Barrow, who put together some of the British label Trojan’s best collections.

The practice of dub reggae (still alive and well in the work of Mad Professor, Adrian Sherwood, and legions of rap and hip hop DJs) is deconstructionist in the true sense of the word. Not only is it about “taking something apart,” the most simplistic definition of deconstruction; it’s about seizing on weak points, points of transition, points of contradiction or double meaning that songs unwittingly contain. In literary studies, where deconstructionism gained currency as a critical idea, puns and wordplay are often used to point out the provisionality and fundamental arbitrariness of language. Similarly, dub producers emphasize structurally significant and potentially contradictory elements of a reggae tune in order to reveal things about it; they draw out and highlight the inner weirdness that already lurks, just waiting to be cast in the right light. Isolate a seemingly normal bass line and suddenly, without its other instrument friends there to back it up, it sounds awkward, bizarre, deranged. All it takes is the flick of a switch and–presto!–a sweet song can seem awful strange.

In contrast, Brise-Glace build from the ground up. In fact, at certain strategic points they keep quite close to the ground: the final five minutes of the 11-minute “Restrained From Do and Will Not (Leave)” hover just around the inaudible range, making it seem like something’s wrong with your stereo. Stunningly packaged, When in Vanitas . . . is the experimental head birth of Chicago guitarist/composer Jim O’Rourke. The ensemble features O’Rourke and Dylan Posa on guitars, Darin Gray on bass, and Thymme Jones on drums, but it rarely sits up and barks like a normal “band.” In a most dublike move, O’Rourke constructed most of the disc’s basic tracks by cutting up and reassembling Jones’s heavy, John Bonham-esque pounding. On “Neither Yield Nor Reap,” the disc’s incisive opener, the beat suddenly doubles back on itself and slips out of phase like a misregistered print. Above, underneath, behind, and around the pulses (which disappear for long stretches), Brise-Glace’s many layers are piled on and lifted off; midway through the 24- minute “One Syntactical Unit,” an appropriated blues song muffledly emerges and sinks as if drowning in the mix. Shortwave radio, underamped rhythm guitar, insistent organ notes, rubbery percussion, and inexplicable noises swirl together in a delicious, at times butt-whompin’, out-rock/rock-out goulash. When in Vanitas . . . recalls classic 70s vanguard rock groups such as Can and This Heat, but it speaks through contemporary teeth.

Tortoise, too, comes in a lovingly designed package: a gorgeous cardboard case with a little dissected tortoise printed in bright red on the CD itself. More bandlike and tune-driven than Brise-Glace, Tortoise use a king-size instrumental armada that includes analog synthesizers, Moogs, organ, various and sundry percussion instruments, and a television set. These augment the backbone of the group, drummer John McEntire (the sticks behind much of the most engaging Windy City extended rock, from Gastr del Sol to the Sea and Cake), and twin bassists Bundy K. Brown and Doug McCombs. (Rounding out the band are Dan Bitney and John Herndon.) Sometimes one bass ventures into the high register to graze a riff while the other lays on a deep ostinato throb; on other pieces, the two entwine like knots in thick rope. Superimposed ambiences and spiky sound spaces are exposed and foreclosed, blanketed by hums, whispers, and mechanical purrs. Direct dub references occasionally appear: the breezy “Night Air” includes melodica, recalling dub’s linear minimalist extraordinaire Augustus Pablo; “Onions Wrapped in Rubber” has an echoing drum thud not unlike the sound preferred by Lee “Scratch” Perry; “Spiderwebbed” starts out with a sneaky bass line reminiscent of Jah Wobble.

Dub is primarily the studio art of audio mixology, but Tortoise have a lot of straight performance verve going for them–indeed, that’s what makes ’em a treat live. What should be interesting is to hear what happens when Tortoise get the real dub treatment, since their next release is slated to be a different version of Tortoise, remixed by Steve Albini, Rick Brown, Bundy K. Brown, John McEntire, Casey Rice, and Jim O’Rourke. Even if these producers recast the music in a way that sounds nothing like dub, as is likely, underlying the project is a refusal to accept a definitive or “final” version. That’s the deconstructionist art of recycling, reusing, and reconfiguring. The dub credo.