Of man, in my humblest opinion, The Alfa 156 is the sweetest car I've known. Would you mind posting some pics of the empeg sitting in that glorious dash? And which engine is it? I think the 2.0 coupled to the selespeed box is magical.
As for your question, I think Tony is right, no door speaker will be able to emulate a subwoofer. In my opinion, such a great ride totally deserves a subwoofer, even a small one. That JL Audio mentioned, with a very well designed enclosure, would add just the needed kick... And Tony is right about a stealth enclosure, too... regrettably, JL only produces stealthboxes for american market cars, so no stealth box fot the Alfa, but you could do it by youself, or have the guys at your sound shop make it...
Good luck, good car