It's hard to replace any of the factory speakers without mods. The two dash speakers are two different shapes. But with some plumbers strap and removal of some foam 4 1/2 inch speakers will fit... Got some in my Brat. The doors are 5" speakers and not many MFG's make those. With a small spacer you can get 5 1/4's to fit. If you wanna go 6 1/2 you need a slightly more ugly 1 inch spacer.
If you do change any of your speakers at the very least run new ground wire if not new speaker wire. The stock Sube uses a common ground and none of the new decks or speakers are designed for that and will either make them sound like crap of just won't work.
I think in brats the door speakers were the optional ones.. but may have been the dash..
Mike