The box designed for the car is a deadly idea, wasn't aware one could be got. In regards to your question a 200 watt 2 channel amp would be more than sufficient for the sub, but considering you have uprated all the speakers it would really be worth while getting a 4 channel amp to run the speakers as well as the sub, even with just the speakers running them with an external amplifier will make a huge difference to audio quality, I currently have a vibe 4 channel 400w amp running front alpines 2 6x9 speakers in rear and a jbl 12" sub and it is more than enough imo, previously ran sub on separate amp and could not detect any difference not just thump thump dance