I was going to reply that you should upgrade the head unit and add good quality amps before replacing the speakers, but it seems you are on the right track.

. . .

I ran all Alpine gear in my 300E and it sounded excellent with the stock speakers (~4", 120mm). I used a 4-channel amp for the stock dash and rear shelf speakers, bridged 2-channel for a 10" sub in a ported box in the trunk, Alpine head unit. Sounded great.[/quote]

I figured that might be the outcome when I did this. I wish I had figured that out before I went through two rounds of speaker "upgrades" in my
CE. It just did not sound good until I added an amp. I'll let you know how this works out.

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1994 E500 199/Gray 82k
84 300D (Salty) Orient Red/Palomino 141k
88 300CE (Ersatzhammer) 904/Java 163k -- Turbo Technics twin turbo kit, AMG Gen I body kit, Sportline steering box and steering wheel, Sportline/Eibach /Bilstein Sport/500E suspension, Quaife LSD in 210 mm diff case, Silver Arrow brakes.
88 300CE Brabus 3.6 199 on Black 44k