Pretty well all OEM audio equipment is sub standard/doesn't reflect the price of the car, particularly speakers probably because they are hidden and it's always been a sore spot with me this penny pinching by car makers that is now considered standard acceptable practice. If you want to improve the sound of your audio system the speakers are probably the best place to start but do your research first as car audio makers supply absurd and unbelievable output figure for speakers. "Max output" is is as much use as a chocolate tea pot, the only ones that matter are RMS or DIN references. Do not buy co-axial speakers, it's components you need for the Foz and there are loads of really good offerings out there just check on the correct size as the Forester has fairly small units. Unfortunately, no matter how good the speaker, because of the rather noisy nature of the Foz, most frequencies below 100Hz will be swallowed up hence the need for a sub to make that back up without straining the door drivers. You should notice a big improvement in sound quality by doing this and even the factory head will sound a lot better but even that is pretty lame so decide what you want, CD, USB, iPod, SD, blue tooth etc, it's all out there. The other factor is sound quality, if that's more important to you than say bass boost (Pioneer/Sony), vast numbers of buttons (Panasonic/JVC/Sony/Pioneer) or sound quality (Alpine, Brekker, Blaupunkt). These are just a few examples that came out the top of my head but there is much more to discover pror to opening your wallet.
The factory installed wiring is perfectly adequate so retrofitting is fairly easy once the sizes are correct but be aware of the new speaker depth because the factory drivers have cheap small and shallow magnets, the better components will have bigger magnets which might interfere with the window so look in the correct section on the forum front page as there are plenty of posts on how to make spacers easily.
As for subs, if you don't want to lose the boot replace the standard under seat piece of crap with one of any number of excellent "kegs" available which will make the OEM part seem puny and a waste of space. This is pretty well what is said above but expanded a little.
When the 6x9 "sub" in the back of my 2004 got disabled as a result of my head unit upgrade, I noticed absolutely no difference.
