I've had a look around and can't seem to find the answer.
Does anybody know if I can bin my standard subs from the back door and use the standard amp to power an aftermarket sub box?

Do you mean bin the loudspeakers only and retain the amplifer in the back door to drive aftermarket sub box, I don't see why not but it will be small like an 8 inch.

Yes exactly that. Why will it have to be small?
My standard subs a all ripped and sound crap but I have a JBL sub box kicking about in the garage so I just wanted to stick that in the boot and use the standard amp to power it.

Because the door amp is only around 40-45 watt, large speakers need power tp move them. Why not just replace the existing speakers as they are standard full range so will handle low-ish as well high frequencies with something similar they are available, but there's nothing to stop you from running a pair of wires from the amp to your spare boxed speaker for a try out the worst u can do is blow the amp. :frown:

Or do what I did and have a 12inch sub loudspeaker in a box in the boot and a 200 watt amp under the passenger seat then u will have a sub boom box..

I went to my local car factors with one of the old speakers and found a set that were the same diameter.

These were not sub woofers, but decent door speakers that would handle some base.

I then fitted them in place of the broken standard speakers.

The only issue was that the standard Amp fitted onto the magnets origionally but they would not fit now as the magnets were bigger.

I have temporarily wrapped it up in a couple of carrier bags to stop moisture from getting to it and rested it in the door.

Does the job and sounds better than it used to.

If you have a more powerful amp I dont see why you could not fit it in the boot somewhere and find the speaker wires and splice into them.

I'll try wiring it straight in and see how it sounds otherwise I'll run a separate amp, just didn't fancy messing about fitting an after market amp when there is already one fitted.