Salt-n-Pepa walk thin lines between all sorts of things: pop schmaltz and gangsta vacuity, female independence and female stereotype, substance and preachiness, self-determination and manipulation. That they've done all this and remained rap's most successful female group is important. They've never put out a coherent album and probably never will; but they have created great singles ("Push It," "Expression," "Let's Talk About Sex") and will probably continue to do so for the foreseeable future, particularly since their latest--the steamy sex romp "Shoop"--was written and produced without their longtime semi-Svengali Herby Azor. Their cataclysmic pairing with En Vogue for Azor's "Whatta Man," on the other hand, shows that the women aren't too proud to take a good song and run with it, either. I've never seen the group live, but a friend in California who saw the current tour says they've got great sound (unusual for rap concerts) and a steamy show. Coheadliner R. Kelly, a Chicagoan, essays a potent black pop that lyrically and emotionally concerns itself almost exclusively with what goes on between the sheets. Yawn if you want to, but this very young writer-producer-performer has an album approaching triple platinum and a single, "Bump n' Grind," that's topped Billboard's R&B singles charts for 12 weeks. That makes it the longest-running number-one since the magazine restarted its R&B chart in 1965, breaking the record set by Whitney Houston's massive "I Will Always Love You" last year. Xscape (see Spot Check) and Kid Capri open. Sunday and Monday, 7 PM, pavilion, University of Illinois at Chicago, 1140 W. Harrison; 413-5740 or 559-1212.