whoa!....I'd sell off the JL amp, what you had is fine, you'll have more power using the 4 channel amp bridged to the dyns, as for the tweeters buy a set you like and play around with the placement for a couple weeks, you'll know the sweet spot when you hit it. not to many Saab drivers to be able to tell you what tweet placement worked for them.
I'd also ditch 6x9's and leave the holes bare so more sub sound comes into the cabin this way you may not need to upgrade the sub amp.

then with all the money you saved and got for the JL amp get someone to build you a fiberglass encloser for your sub in the corner.

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