[Cunningr]

My suggestion for economical upgrade is swap the factory hu, change factory amp to the 6 channel 996 cabriolet amp, and new dash speakers.
Use the rca out on the new hu metra has a install kit so its plug and play. This will add eq, bluetooth most likely and few other options depending on hu.

The 6 channel cab hu will allow the fader to be used and as mentioned open up the bass from the doors.

If you really wanted to do it nice, get the pioneer prs80 hu run it network mode. Use seperate 4 inch mid tweeter dash, run tweeter off hu unit amp for high freq. run mid and low to factory 6 channel amp. I ran this configuration for awhile and sounded very good.

The pioneer hu in network mode has active cross overs and time alighnment functions. Has really nice eq, i think for $300 best bang for the buck in a head unit.