As Adrian says...

If you are going to go further than simply replacing what is supplied by MMC I'd consider the following:

1. Replace the 2 x 4"(100mm) units at the back with 6" units.
2. Fit tweeters at the front, either in the upper doors or ends of the dash
3. Fit a sub woofer in the passenger footwell.
4. Replace the head unit with a new Pioneer one: if it is possible one with a satellite radio receiver. In Europe DAB is the way forward for car audio, in N America it seems DAB hasn't caught on but satellite radio has.

Let us know what you actually do!

Peter

Peter,
66, 2016 Porsche Boxster S
No longer driving Tarka, the 2014 Plus 8...