Right I've fitted my self an Alpine headunit so my iphone now works GREAT with the car, next is the speakers as there not the clearest of sound and seems to be lacking base, anyone know the sizes and how easy they are to remove.
if it's like the GTV, the standard door speakers/woofers are oval, like 5x7". but with a new baffle (ie: replace the plastic speaker holder with an mdf wooden ring), you should be able to fit most 5.25", and probably a lot of 6.5", depending upon the size of the basket/magnet.
sound deadening and sealing up the doors will greatly improve (mid)bass response. so too powering the speakers from an amplifier. ultimately, only a subwoofer can provide good (sub)bass. though it's more difficult in a softtop due to higher roadnoise, and somewhat impossible to get loud subbass with the roof down whilst driving.