It's very rare to hear two musicians who play together with the care and sensitivity of saxophonist Urs Leimgruber and drummer Fritz Hauser. They're musicians with big ears: they listen carefully and respond quickly to each other's changing thrusts in dynamics, sound, and momentum. Theirs is a flowing, carefully crafted music, full of detail, whether wholly improvised or predesigned (Among the delights on their Hart Art CD L'enigmatique is a startlingly rich sequel to Ellington's railroad-train songs). Leimgruber is something of an eclectic; sometimes he'll evoke Roscoe Mitchell's care for the properties of overtone sounds, then he'll slip into Evan Parker-like lines of harsh fragments. Though he's inclined toward abstraction, there's an overall sense of drama about his playing. Hauser is a wonder. He conceives of the drum kit--snares, tom-toms, bass drum, cymabls--as a full orchestra that creates the loudest and softest sounds possible, as well as the highest and lowest sounds from cymbal overtones to drum rumbles. His sensitivity to shading of sounds and dynamics recalls that of the late Steve McCall, Chicago's own modern percussion giant, though he's otherwise quite dissimilar. When Hauser chooses to swing, there's marvelous uplift in the African- and Oriental-sounding rhythms he invents. This subtle Swiss pair forms the first half of a transatlantic weekend at Southend Musicworks; tomorrow night the broader-humored Maarten Altena Ensemble from Holland led by theatrical bassist-composer Altena, including the remarkable trombonist Wolter Wierbos, takes the stage. Friday, 8 PM, 1313 S. Wabash; 939-2848.