you do not need a pac adapter. put some coaxials in the fronts and use your amp for those

 

With hardly any knowledge of this stuff I hadn't looked into coaxial speakers. I don't know if I would want to get rid of the subs in the doors for those but would I be able to replace the rear speakers with coaxials and an amp?

Any recommendations on a head unit based on what I'm looking for?

 

upgrade the front speakers and leave the shaker door subs

imo upgrading the rears is the most *** backwards thing you could do