Check out the thread below in the 2 series forum. A ton of us have replaced stock with the Eton/JL Audio combo - easy swap over with great visual guides...results are night and day improvement over stock HK system - all in for about £400 too (speakers, harnesses, tools, dynamat etc..)
Dr. Strangelove, or How I Learned to Stop Worrying & Live w the Harmon Kardon System
http://www.2addicts.com/forums/showthread.php?t=1236238