It's called a composers' orchestra, but a principal attraction of this 17-piece crew is its terrific soloists. About two decades ago, when Europe was getting into its jazz renaissance, London was home to a group of extraordinary players who took the discoveries of the free-jazz revolution to logical and illogical extremes. Bassist-composer Barry Guy, a virtuoso sound explorer, formed the London Jazz Composers Orchestra in 1970 to capture and exhibit all this wild activity. The group includes innovators like soprano and tenor saxist Evan Parker, the Sonny Rollins from hell with his coarse, splintered sounds and inhumanly long lines, and virtuoso trombonist Paul Rutherford, the very soul of unreason--his concept of found sounds and unformed forms makes John Cage seem like a right-wing conservative. Also on the band are altoist Trevor Watts, who finds dancing rhythms in the most far-out places, and the sensitive yet passionate Paul Lytton, a pioneer of electronics in percussion. The orchestra makes its American debut in Chicago this week with two concerts. Wednesday's concert, titled "Subsets and Intrasections," will be purely improvised by soloists and small group combinations from the band; hopefully the Parker-Guy-Lytton trio, which played an extraordinary concert at Link's Hall in 1986, will be reprised. On Thursday the full ensemble will play extended works by Guy, who recalls Chicago's own Muhal Richard Abrams in his ability to compose settings that unite brilliant, disparate soloists into multicolored activity. These concerts may well be the high points of the season. Wednesday, 7:30 PM, Southend Musicworks, 1313 S. Wabash; 939-2848. Thursday, 7:30 PM, Pick-Staiger Concert Hall, Northwestern University, 1977 Sheridan, Evanston; 902-1500 or 708-491-5441.