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.
