Basically Bach, the enterprising period-instrument ensemble, deserves an “A” for chutzpah for attempting to subject Mozart’s greatest and most complex symphony to the “authenticity” treatment. The profound beauty and formalism of the Jupiter Symphony depend heavily on its contrasts and reconciliations, which can be best expressed by a large modern orchestra. (Even by 1788, the year of his final symphonic trilogy, Mozart knew what a midsize orchestra staffed with a strong wind section was capable of.) I’m skeptical about how the ambitiously scaled symphony might sound in the hands of a 30-instrument group such as Basically Bach. But Daniel V. Robinson, the group’s knowledgeable director, has achieved convincing if not exactly “authentic” results in the past, and he may have a persuasive case. Two other pieces on the program–Horn Concerto no. 3 in E-flat and the concerto aria “Ah, lo previdi!”–were indeed written for smaller (court) orchestras. And the solo horn passages in the concerto will be played on a natural horn, as Mozart intended. Unlike its modern relative, the natural horn has no valves; pitches are produced through the player’s puckering lips and by hand movement in the horn’s bell. The soloist is Lowell Greer, a horn specialist from the University of Michigan. The aria, to be sung by soprano Christine Brandes, is a quasi-Handelian showstopper that conveys Andromeda’s anguish at the slaying of her lover. Tonight, 8 PM, Scottish Rite Cathedral, 935 N. Dearborn. Saturday, 8 PM, First Congregational Church of Evanston, 1445 Hinman, Evanston. Sunday, 7 PM, First United Church of Oak Park, 848 Lake, Oak Park; 334-2800.