You’ve got to hand it to Johnny Reinhard. His American Festival of Microtonal Music keeps the varied sound of alternative tunings coursing through Manhattan in a flow remarkable for its quantity, if not always quality. And every now and then, amid the quarter-tone cello pieces and homemade instrument demonstrations, he pulls off a concert so stunning as to outclass the “Hooray for Modernism” festivals uptown that garner so much more publicity. Such was the enlightening March 8 concert of the Flux Quartet.

For the Flux (Tom Chiu, first violin; Max Mandel, viola; Dave Eggar, cello; and Conrad Harris substituting on second violin for Jesse Mills on this concert) is becoming the quartet that thoughtful new-music lovers had hoped the Kronos would be: catholic in repertoire, dynamic in performance, and not stooping to theatrics or watered-down Astor Piazzolisms to gain an audience. An American Arditti, in other words, not speeding up Morton Feldman’s Second Quartet to four hours for fear of arm fatigue (as the Kronos did), but stretching it to six, on their resplendently meditative new Mode recording, in accordance with Feldman’s apparent desires. And whether they found their own microtonal repertoire or had it given them, this weighty, varied concert flouted the truism that microtonality has produced a lot of good theory but little good music.

The great divide that keeps microtonalists sniping at each other is equal temperament (equal division of the octave into 31, 53, even 96 or more steps) versus just intonation (unequal division for maximum consonance with fewer pitches). The Flux played strong examples of each, by Mexican pioneer Julian Carrillo and American patriarch Ben Johnston. Carrillo made a career off what he called “the Thirteenth Tone,” and I had heretofore found his music rather gridlike and unimaginative. But 2 Bosquejos was Romantic in texture and gesture, and to hear the Flux play parallel chords moving through eighths of a half-step, always in tune and in that otherwise conventional context, gave my ear a thrilling twist.

Johnston’s Fifth Quartet (out of 10 so far) was a more complex pleasure. Coming after his fiendishly difficult but widely popular Fourth Quartet, based on “Amazing Grace,” it is a less extroverted but still very demanding work; the Kronos decided that it was too hard to attempt, though he wrote it for them. Based on the American folk song “Lonesome Valley,” the piece opened cloudily and in different tempos at once with that tune played over rocking fifths in the cello. Twice it grew through textures of Ivesian layering to hectic climaxes, and along the way gave Chiu more than a few opportunities to slide through melodies of tiny increments involving the 13th harmonic. More intuitively written than some of Johnston’s later quartets, this one has deep roots in his Southern upbringing, and is one of the most soulful works of a master too neglected in New York.

Not quite so rare (because Uptowners champion him) was the Fifth Quartet of the Italian recluse Giacinto Scelsi. The piece was single-mindedly, not to say minimalistically, focused on creating via strings the noise envelope of a struck gong, each gesture beginning with a loud pluck, going to a thick, scratchy crunch, out of which would slowly evolve a purer, glissandoing unison tone. The most heterogeneous work, by contrast, was Reinhard’s own Trespass, using both quarter-tones and a scale based on the 17th harmonic, the latter of which produces a wide array of interval types. After Mandel started playing, Chiu entered the stage sneakily from the audience, as if infiltrating. This wasn’t the first time I’d wished Reinhard’s own music were more serious in intent, for his microtonal expertise is phenomenal, and the piece offered a disunified array of ear-bending tuning displays.

The seemingly non-microtonal Second Quartet of Charles Ives was included here because it was played in extended Pythagorean, pure-fifth tuning in keeping with an alleged “underlying acoustical plan” that Reinhard has discovered in Ives’s notational idiosyncrasies (such as, for example, including a D-sharp and E-flat in the same chord). That’s as may be; we needed no more urgent excuse for a performance as energetic as this, not quite as crisp in the chaotic second movement as I might have liked, but achieving an ecstatic and mystical third-movement climax. My ear’s not good enough to distinguish Pythagorean from vanilla in an atonal context, and I assume we got what was advertised. But in terms of monuments of microtonal music played with consummate power and authority, we got more than anyone could have expected.