Despite what the car audio shops are telling you rear speakers are only good for rear fill, as gIzzE says maybe a bit of mid range. Take with a pinch of salt the advice of people who sell an item you are asking about!
I wouldn't have gone for the Alpine upgrade if I were looking to change out the HU. It's not a bad upgrade but you've hindered your upgrade paths now with that amp!
Have you considered a processor like the JBL MS8? By the time you've changed out the under seats for SWS8's and bought a mid level after market HU you'd probably have spent about the same as an MS8 would cost you.
You could ditch the Alpine amp, run everything off the MS8 using the under seats as mid bass and for a small amount more could add a centre channel as I did. If you want more bass add a discreet amped sub box in the boot. The MS8 probably doesn't put out quite as much power as the Alpine amp but trust me it would be much better owing to it's processing.
Full control, OE look, no issues and the MS8 will be easy to sell on afterwards or keep for your next car
