Rear Door Speakers in 3 Door (Query for AndyMac)

Hi Matt

Yeah you can fit pretty much any amp behind the rear speaker panels in a 3 door A3. There is loads of room in there. I've fitted F400, F407's and there's room to spare. However you are going to be limited as to the sound you can get from that location.
1. The location itself is not great for a balanced sound
2. The feebleness of the hardboard door cards (which are only held in place by spring clips) means you can't really get any level of bass from the rear (which is normally the only benefit of big rear speakers).
If you are amping the rears I would definately suggest amping the fronts as well.
The pre-outs/remote on are accessible from the loom going to the sub (but only the rears). Pin outs are shown in the instruction manual.

Not sure how you'd do that bar building a completely new panel, or building an enclosure behind the panel. Neither of which is an easy task. A sub is a much easier option.

If you got a sub already I would forget the 6 by 9 and get some good speakers for upfront mate. Spend your money upfront where you are the most, driving and not in the back. You will have mad hassle to fit them properly in the door card and you will have an air leak. If you do want to replace the speakers in the back get some the same size or at least the same shape and then you can make an adapter ring out of MDF. trust me I've been there.

good luck