is it easy enough or worth doing?
there is a big loom of wires going to the amp from the deck which confuses me because i thought the amp only powered the sub not all the speakers aswell??

id like to put a better amp in and a better 8 inch sub that actually packs a punch, but will still fit into the parcel shelf position of the factory one.

Try replacing the sub only first up. There's really nothing wrong with the amp if you're going to keep with an 8" sub. Go for a bigger sub/s then you'll need more power.

Replace the crap sub first. There's a few options out there, but not many.