Once again, the Kimmel Center's Fred J. Cooper Memorial Organ emerged from its splendid semi-isolation with revelations at many turns Sunday with the Chamber Orchestra of Philadelphia. Organ recitals have their audience, but recent collaborations have uncovered important but neglected repertoire and perhaps have expanded the organ audience. In Sunday's program of Handel, Josef Rheinberger, and Joseph Jongen (with four different soloists), the big discovery was Stephen Paulus' 1992 Concerto for Solo Organ, Timpani, and Percussion.

It's a terrific piece that would have to rank among the best American organ concertos of the last century, with four hefty movements that strike out in many directions, from elegiac to comic, with equal conviction. The organ made a crazy, explosive first-movement entrance in a piece that found several places of harmonic stability, only to leap away from them in compulsive sorties to everywhere and nowhere. Most entrancing was the second movement, with the organ creating a liturgical atmosphere joined by a sensuously pagan violin solo playing in an intriguingly alien key.

Hunting down other performances on the Internet made me appreciate the care and sensitivity that music director Dirk Brossé and organist Alan Morrison put into a performance that gave no hint of the many possible land mines in this incredibly eventful score. The orchestra, expanded to about 60 pieces, has rarely sounded better.

The other discovery was Jongen's lyrical, pastoral 1924 Hymne Op. 78. Whenever the composer veered toward what we would now call Hollywood predictability, some unexpected chord or color would act as a course correction in a serene journey not unlike the modern works of Einojuhani Rautavaara. Soloist Matt Glandorf drew warm but light timbres from the organ.

Handel's Organ Concerto in G minor Op. 4 No. 1 was an expected pleasure. Perhaps some listeners also knew what they were in for with Rheinberger's Organ Concerto No. 2 Op. 177. The best I could say amid the imposing performance with Jeffrey Brillhart was how much mileage this big, romantic-era piece achieved with no genuine content.

Instead of including the usual bonbon from whatever film or concert work he has written lately, Brossé brought out a revised version of his 1998 violin concerto Black, White and In Between that's not a bonbon of any sort. The first section, with Miho Saegusa as soloist, is a serious, searching piece that really didn't need the elaborate spoken introduction Brossé gave before the performance. Is it possible that in past concerts, Brossé has been selling his composer-self short?