audio upgrades in NC miata


2011 mazda 3 hatchback GT
hello miata owners, i am looking at getting a NC 07 mx5 and was wondering what audio upgrades are possible in these little cars. obviously i want to rid myself of of the old beat up factory stuff in the
one i am looking at now, i do not believe the one i am looking at has bose, although it is a grand touring. a few questions for those with these cars whom have done audio upgrades,

1 what speaker sizes fit in the front door? crutchfield and sonic electronix list 6x8s 6 1/2s and 5 1/4s as all fitting so its confusing me some

2 what size speakers are the ones between the seats? (cannot find info anywhere for this)

3 do the speakers need to be marine speakers because of the convertible top? and

4 how do i install a subwoofer system or some other system for some bass boost in a car this small and still be able to put anything but myself into it?

i am coming out of an 08 3 hatchback with a 1000w single 12 inch sub and 300 watt 4 channel door speaker aftermarket setup, i understand i wont get that system in this car, but as i listen to rock and metal i do want some bass in my music. i know i can keep the 4 way amp as it is capable of doing 2 channel door speaker operation and bridging the rear 2 channels for subwoofer output and has all the adjustablity for that. i am also looking to get away from overbearing kill my eardrum systems and just looking for sound quality that i can enjoy now. i am over the big system phase.

if there is anything else i need to know or look out for let me know as well, i really dont want to chop into the car as much as possible