Personally, I like high current amps....one or two amps that you can run your whole system on. I've got an Xtant 604x with the rear channels bridged to power the subs (electronic crossover built into amp) and it sounds great.

In my M3, I didn't want to sacrifice any trunk space so I put some JL audio free-airs (8IB4 Stealthboxes) in the rear deck as the subs, and left the front sound stage to my a/d/s separates. I hate having "fill" from the back because it sounds a little un-natural, however there is some music now that is designed for dolby type setups (not sure if the Empeg can emulate this).

Anyhow, stick with the basics and don't skimp on interconnects. You may need to apply a dynamat type material to some panels to lower the resonance and make sure to route all wires to minimize noise. I'm sure you know this already.

Those Brax amps are very nice, but you may not be able to tell the difference between a Brax and an Xtant or similar quality amp when moving down the road given that the Mustang doesn't have Lexus like sound deadening.

Good luck!
Wes