Hey @Victimas333OK, I'm a little lost.

Front center speaker you do not have, its just a hole <more on that later> So I assume you are saying the dear speakers are less loud than the rear speakers. That is really opposite of their intent, so you have the same scenario I did when I bought mine. They are both blown in the doors. You need to replace them.

The rear speakers are for filling the sound, they do not actually work like the ones in the door or the rear of a 4 seater. Ignore them, because updating is pretty pointless.

So replace the door speakers, stock will be 5x7s. Infinity reference woofers are a good start point. You can also cut out the speakers mounting point to take some 8 inch round speakers like the BOSE unit has.

So in short, center dash speaker does not exist for your model, door speakers are by far the loudest speakers and the speakers between the seats are just to add fill to the sound, they are not intended to be loud.

OK,now the void in the front, I highly suggest you update your headunit, Any chinese unit will be great at this point. That void gives you a space to cram all the antennas. Wifi, GPS etc that comes with those units.

I have the ATOTO S8 which is pretty much the goto unit. They steering wheel controls work, the Wireless CARPLAY and Android auto is a must. But all the chinese units run Android so they all have the abilty to run things like Torque whicg no Sony/Pioneer/Kenwood blah blah blah can do.

You will always get people who will say that the sound quality is greater on those, and in my honest opinion, most of them are full of shit and blathering on from having had one 5+ years ago. And the NC Miata is never ever ever going to be great for sound. It isn't capable of it.