Wu Man is a peerless virtuoso of the pipa, a Chinese lute-like instrument rarely heard stateside in close quarters, much less in the hands of its greatest living master. On Sunday night, the Shanghai Quartet paired it with classical strings, closing out the Cornell Concert Series with an adventurous program. The results, however, were inconsistent and, at times, baffling. The Shanghai headliners began with three Chinese folksongs — “Yao Dance,” “Shepherd’s Song” and “Harvest Celebration” — beautifully arranged by the quartet’s second violinist Yi-Wen Jiang. The last two songs in particular, both from the southwestern province of Yunnan, varnished the grain of their arranger’s relationship with the music of his homeland, further showcasing the superb technique, dynamic control and finesse that have earned the quartet high regard.