I just upgraded my sound system and I must say there is a HUGE difference between the upgraded sound package offered by Subaru and the setup I got. If you have some money to spend on it, this would be my recommendation.
Head Unit - Pioneer Premier 940MP (plays MP3/WMA)
Cd Changer - Pioneer Premier CDX-PD6 (indash just like stock)
Front Speakers - Pioneer Premier TS-C604 (component)
Back Speakers - (upgraded stock speakers ie dont change)
Amp - Sony Xplode 5Channel 5150gsx (880Watts)
Sup - Alpine 12 inch Type S
Sub Box - Q logic single sealed box.
Obviously there are other options for all of the above and Im sure there are DEALS to be had out there. ( Maybe get the Pioneer 9400MP instread ) but i wanted to go Premier.
This setup sounds very clear and the best thing is the options on the 940MP. You can change EVERYTHING.
Hope this helps someone.
Let me know if I made a huge mistake.. I can still return most of it