It's no trade secret, the stock radio in the WRX is pretty weak. However, Subaru has recently been making some nicer radios that have frequently been swapped into other vehicles. In this case, my friend has tracked down an HD Radio w/Satellite option from a 2013 Subaru Forester 2.5X Limited.
H621SSC300 - Forester HD/SAT Kit
H621SSC001 - WRX HD/SAT Kit