Replace the stock head unit.
Take your pick of brands. I'm partial to Alpine & Pioneer. Pioneer packs the better feature set with still pretty good audio quality perks.
The new head unit (and steering wheel controls) will govern your usb connections/steering wheel control integration/hands free calling. I would just run a new usb cable to wherever you want.
Speakers are pretty nice stock actually. I'd just use the head units internal amps unless you want some more power after you install the head unit.
Check out Subaru radio and radio wiring harness information for all the wiring and steering wheel control details. I would use one of his harnesses. Would make the install mostly plug and play minus running accessory wires.