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.