2) Since I've got an amp in the boot already, I could stop using the rear speakers and use the existing amp to power a sub.

If you have enough spare power in the amp, you can put a front and a rear speaker in parallel on one channel of the amp. I did that with a 4-channel amp (bridging the remaining two channels to driver the sub) and I'm pretty happy with the result. One drawback is that you lose the fader control but for me the sound image is still in front, so I don't need it.

Borislav