Paul Wall, a big man in a bigger T-shirt, is tapping at his Sidekick when the car arrives outside his Manhattan hotel. He looks up and flashes his glittering grille, eyeing the ride with something like amusement.

"A Subaru?" says Wall, the Houston rapper whose slow-sizzle major-label debut, The Peoples Champ, was released last month and marched directly to the top of the charts. "I was thinking Bentley or...