In theory yes, but the results will be awful. The Bose system in the A3/S3 is pretty dire (as many on here will agree, see ICE forum), and the main problem is upgrading it really means stripping everything out, speakers, amps, sub & HU. It's ot all bad as you can sell the individual components on ebay probably for £300+ if you sell them seperately, as there is a growing market for spares as speakers & subs tend to blow quite easily and the HU has inherent problems of it's own.
It's actually easier to upgrade the non-Bose system as the speakers are standard 4 ohm (Bose are 2ohm, 4" units). The Bose system uses the same tweeters in the front doors as the non Bose.
Install new HU, get some nice 5.25" components up front and put in a decent sub and you're away.
