Better question: What do YOU think needs improvement in the current system? If nothing is bothering you, then a replacement OEM speaker is the cheapest and most sensible option.

Adding an aftermarket amp will involve some PITA wiring and weird brackets/boxes. OEM kicker amp in trunk and upgraded HU will give you decent improvement in sound quality with the least amount of install effort. Crutchfield & Metra have harnesses and frames to make some aftermarket HUs fit flush and work with steering controls and backup camera.

 

I'd like improved eq controls, sound quality, and volume (nothing crazy on the volume). I've also come across the Idoing head unit which seems like the way to go. All plug and play without the need for additional harnesses. I'm just not sure how that would then integrate with the existing HK system if I were to install new front door speakers and an amp. I don't need a sub but I do want a crisp/clean/punchy sounding system.

 

I guess my hangup is.. I'd like better sound quality/performance out of my front speakers.. and I'm looking for the most efficient way to accomplish this. Maybe I'll talk to a crutchfield technician.