Upgrading parcel shelf speakers

[C.A.R.]

I've got a crackle coming from the back of the car when listening to louder music, have just upgraded the fronts to some nice Kenwood 6.5" jobbies and wondered if you can do similar to the back?

I don't want to stick anything too heavy in there as I know the shelf can sag with too much weight on it.

I tried to remove one of the factory speakers from the little plastic enclosure but it wouldn't budge. Is it glued in? It would be nice to retain those plastic things but upgrade the speakers as they probably offer some protection from the crap in the boot...