I'm considering the same thing, that is adding a 4 chanel amp and upgrading the speakers. I added a powered subwoofer last week and the "domino effect" has begun.
I plan to leave the tweeters in place but run the amp to the new 2 ways going in the doors and out back. I'll be using an amp with speaker level inputs though. I figure the amplified door speakers will be louder than the stock tweeters. If I can't adjust it to my tastes, then I'll consider disconnecting them.
I've also considered just doing door speakers w/a 2 channel amp and leaving the rear speakers alone. It's all debatable, but I already have a 4 channel amp in storage.
