EOS ORCHESTRA. Music by H.K. Gruber, Hanns Eisler and Kurt Weill. Kurt Nikkanen and Joel Pitchon, violins. Ann Panagulias, soprano. H.K. Gruber, conductor. Thursday night. Ethical Culture Society, 2 W. 64th St., Manhattan.

THE EOS ORCHESTRA, a doggedly inventive ensemble that has until now been the private fiefdom of its founder and conductor Jonathan Sheffer, invited an outside agitator to its podium Thursday: the Viennese professor of irreverence H.K. Gruber.

A composer, conductor, chansonnier and all- around character, Gruber has the demeanor of a learned jester, an Abbie Hoffman of the concert hall. He has written an operatic "pig-tale" for children, "Gloria von Jaxtberg"; a wildly sinister and playful piece called "Frankenstein!!" involving toy instruments and the composer's own mordantly cabaret-style singing; and "Gomorra," a stage work of raucous satire.

Yet the Gruber who took the podium and the microphone at the Ethical Culture Society was an entertaining but more muted presence than his catalog suggests. With Eos, he and violinist Kurt Nikkanen performed his violin concerto "Nebelsteinmusik," a rickety structure made of 12-tone techniques and naive tunes that plays with the clash of sentiment and rigor. "Photo-Fit Pictures" tamely gathered strands of Bach, Bartok and cool jazz into a set of orchestral variations that eventually reveal a theme. Neither piece quickened the pulse.

Gruber also brought the music of his left-leaning idols, Kurt Weill and Hanns Eisler, the relentlessly Communist composer of proletarian anthems and robust agitprop. Eos' tightly strung performances of Weill's almost never-heard soliloquy for voice, violin and orchestra "Der Neue Orpheus" (with a stinging performance by soprano Ann Panagulias) and Eisler's only moderately hectoring "Kleine Sinfonie" formed the concert's backbone.

All this would have made the concert valuable; the finale made it memorable.

Expertise in Weill's songs now come in a variety of hues, from the classic suavity of Andrea Marcovicci to the neo-Lenya growl of Ute Lemper and the ravaged utterances of Marianne Faithfull. But none of those chanteuses sound more lean, authentic and funny than Gruber, who slices through to the song's brittle core with his unpretty voice and exact enunciation. There is comedy in his voice, and his roguish Viennese r's roll into merry little bursts of mortar fire.

Gruber mostly sang in German, conducting bandleader-style, waving one distracted hand behind him. In "l-Musik," ("Oil Music") he evinced rousing rage in the anti-corporate refrain ("Shell! Shell! Shell!"). In "Berlin Im Licht," he summoned the era's stirring pride in a newly electrified and suddenly brilliant quarter of the city. But he also ventured into English-and not just any English, but the "Cowboy Song" from "Johnny Johnson," in which he sounded much as Weill must have done, like a Mitteleuropischen cowpoke. Here's hoping he rides this way again.