I'm rarely moved by publicity material, but a quote in the Amati Quartet's demands notice: "A concert by the Amati Quartet," reads the personal, hand-signed letter, "was one of the most thrilling concerts I have ever attended." The author? No less than master violinist Yehudi Menuhin. That's serious backing, in string-music terms. And indeed, the Zurich-based quartet has earned much praise over the course of a 15-year career playing the full gamut of music from the classical and romantic canon as well as certain parts of the contemporary repertoire. A recent recording of the last three of Bela Bartok's six jagged gemstones, released on the Swiss Divox label, finds the Amati in brilliant form: their hard-hitting, clear, full ensemble sound might in fact rival the classic readings of Bartok by the Juilliard Quartet. Willy Zimmerman and Katarzyna Nawrotek play first and second violins, respectively; Nicolas Corti plays viola; and the newest member (now with the group for a year and a half) is cellist Claudius Herrmann. To give a sense of the quartet's orientation, the Amati's other recordings have ranged from Haydn and Mozart through Cesar Franck, Gabriel Faure, Maurice Ravel, and Karol Szymanowski to 20th-century Swiss composers Hermann Haller, Vladimir Vogel, and Rudolph Kelterborn. In the first of two admission-free concerts, Amati will feature a cross section of their musical concerns. At the Cultural Center they'll play the 80-year-old Haller's Quartet for Strings no. 3, composed for Amati, and the third of Beethoven's three Rasoumovsky string quartets in C major. At Swissotel later that same day, they'll again play the Haller, adding quartets by Mozart and Szymanowski and Franz Schubert's Quartet no. 14, Death and the Maiden. Thursday, April 25, 12:15 PM, Preston Bradley Hall, Chicago Cultural Center, 78 E. Washington; 744-6630 or 346-3278. Thursday, April 25, 7:30 PM, Alpine Ballroom, Swissotel, 323 E. Wacker; 561-2424.