Although I don't know for sure who makes them, the stock speakers do sound decent. I wouldn't worry about "upgrading" to Bose as Bose sucks. Ok, so I'm biased because I work in high end audio / home theater but it is truly a myth that Bose actually produces a quality product. There is a good reason why they never post any technical information on their equipment. See this link if you're interested.
http://www.intellexual.net/bose.html
Every Bose car system I've heard has sounded pretty awful. Stock is definitely preferred for a more even, balanced sound. Upgrading from there with quality replacements will also yield good results.
The best upgrade in sound for the buck is to change the cassette stock unit with the CD unit that is standard in other countries. They call it the Audio 10 and it has way more bass and treble adjustment and gives you a better, fuller sound. Also since it's an MB radio, it works with the steering wheel controls, dash display and CD changer. The Audio 10 is made by Becker in Germany, the stock radio is made by Alpine in China.
You can find them for between 100-200 euros on german ebay (ebay.de) or from Steve, the europimp on the forum.
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