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Thanks to Cryptoboy for sending me this link for the speaker mounting plates.
The Bose speakers have an integral amp - when putting together a replacement we just let the head unit drive them? With a crossover mounted on the backside of the mounting plate?
If you let just the head-unit drive the front aftermarket speakers, you'll have very little bass. The factory head-unit is designed to just run the 3.5" twiddler.
Chances are, the cross-over you mention is to cut off frequencies to the tweeter.