That setup wouldn't sound bad, but whats your budget?
That wouldn't sound nearly as nice though. IMHO, if you are already spending the time to replace the door speakers, amplify them.It would have been easier to purchase component speakers up front only(like MB Quart) with lower power requirements so that you could run off the head unit only. I would just leave the stock rears as filler and just have an Alpine Monoblock amp (with built-in LOC) to control an Alpine free air sub. This would have been the easier set-up IMO.