Our 95 E420's sound system is easily the best out of all the other cars we've owned.

including:
83 300CD, various Toyotas (Crowns, Corollas, Windom, Camry), 92 230E, and 96 E320 (it had a Pioneer headunit and MB speakers), Nissan Cefiro (Infiniti I30/Nissan Maxima).

It still isn't quite as good as my friend's Bose in the 99/98 Audi A6 Quattro 2.8

I think the E420's sound system isn't that powerful and once played loud the door speakers start rattling some things in the door pockets. But it's definitely clear and doesn't sound bright or muffled (like most stock stereos)

but then again sound quality is subjective..

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2008 BMW 335i Coupe