I just replaced my infinity head unit w/ a new one from crutchfield and went ahead and opted for the interface to retain the factory amp. I also just added a sony 600w amp pushing 2 Rockford Fosgate 10" subs under the rear seat.

The subs sound good and overall sound quality is good but I think the front driver tweeter is blown or on the fritz. I hear some crackling up there, and I haven't really pushed them that hard.

Do the factory amps have a crossover to prevent the low freq. from getting to the tweeters or are they vulnerable when you replace the head unit?

I'm not opposed to replacing the factory speakers but It kinda makes that interface that I bought w/ the Head unit a waste because I'd be bypassing the amp.

 

The factory speakers stink anyway. I have a blown 1 already with a complete stock system. There is a radio shop local to me that will replace the speakers adding a harness so it still runs with all the factory wiring. Im sure you could search the net to find said adapter to make life easy for yourself.

 

I'm not sure where the factory crossover is. I thought that it would be in the amp if the input side of the amp only has 4 channels (2 front, 2 rear).

Anyway, go to Best Buy and get a pair of Infinity 3.5" Reference series speakers. I added them to my non-infinity system ans they sound great. They came with a resister to block the dangerous bass frequencies.