You might want to check out AudioControl's new LCQ-1. Availability might be an issue, but I just got mine installed with aftermarket front speakers and the difference is AMAZING. I'm extremely happy with it and I'm quite the audiophile. Has summed inputs and isn't too difficult to hook up to the Sony amp. Only $300.
Hate to disagree with a few of you, but the Sony system in the Focus is definitely one of the more mediocre OEM systems I've had, especially in the bass department. Different strokes. [

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I have a stereo install thread floating around here somewhere with detailed pics. I'm in the process of adding new pics showing the LCQ-1 install.
Tony