Forget this capacitor mod if you have 750 USD.

1,)
Dampen the doors and the rear wall with alubutyl and foam. 100-120 USD

2.)
Replace the stock hu with an Alpine Imprint unit. For example a IDA-X305 and add a PXA-H100 processor and the microphone, so you can do an automatic setup.
500 USD

3.)
Buy some good tweeters that will fit into the a-pillow. Then you'll have a much better staging.
100 USD

4.)
30 USD for wiring and additional installtion stuff

5.)
The new tweeters will looped back through the hu amp. So you will not need an additional amp, or new loudspeakers. The result is a very good sounding hifi system with correct time alignment, eq and staging.

If you have detailed questions, don't hesitate to ask.

Regards
Markus