Mozart wrote that he put "long and arduous work" into his six string quartets dedicated to Haydn -- a rare statement from a composer who wrote easily and fluently. The dedication, perhaps, masked an effort by the upstart Viennese hotshot to outdo the already legendary veteran: Mozart's quartets have a nervous quality unusual in his music, combined with quite a complement of really striking effects. Played and heard properly, they're so intense to listen to that the usual single-disc grouping of three of the six quartets is a bit of an ordeal; one of the many attractions of this disc by the Mozarteum Quartet of Salzburg is that two of the six quartets dedicated to Haydn are rounded out with an earlier and much lighter Mozart quartet, the String Quartet in G major, K. 80, the very first one he wrote. The Mozarteum Quartet captures the tension in the String Quartet in D minor, K. 421, and String Quartet in C major, K. 465 ("Dissonance"), perfectly without being melodramatic about it. The quartet avoids making the very sharp-tongued minuet of K. 421 into an extravaganza of stabbing pain, but the tension level is very high. The driving but detailed playing in the outer movements of the two later quartets brings out the many moments of unconventional dissonance -- listen to the beginning of the development section about four minutes into the first movement of K. 421, for example -- in such a way that the famed barely tonal passage at the beginning of K. 465 seems of a piece with the rest of the music in the set, not a one-of-a-kind experiment. The little K. 80 quartet by the 14-year-old Mozart is a delightful finale. A bit unwieldy in its movement sequence (a long Adagio followed by three short movements, the last of which was added later), it is full of daring touches that foreshadow the mature composer. Check out the use of the very top of the range in the two violin parts in the Minuet -- Leopold Mozart blue-penciled this passage, but the Mozarteum Quartet wisely sticks to the young composer's original plan. In both this work and the two from the later set, the group conveys the impression of being in tune with Mozart's goals and intentions -- something that's not easy to do with this composer who seems so simple but gets more intricate every time you look. These are superb performances of Mozart chamber music.