Subwoofers are as much about the enclosure as the speaker. If you're installing 6.5" speakers in your doors, you don't get much control over the enclosures in that situation.

I don't think that 6.5" speakers in your doors would be an appropriate substitute for a genuine subwoofer. No matter how good the speakers are, the doors are not going to help them out.

You should look around and see if anyone makes any custom subwoofer enclosures for that Alfa. It's possible that you could get a 10 in there somewhere without impacting your luggage/passenger space. You'd be surprised how crafty they can get with custom enclosures.

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Tony Fabris