Just to share what i did to my saga. Last year changed my HU to pioneer that supports SD card, cos having a usb drive sticking out is ugly and inconvenient. Installed 6.5" component at the front door, and muted the rear stock speakers. Overall is a huge upgrade, sounded so much better.
After listening over a year, i felt its not good enough. So planning to soundproof front doors, add amp and subwoofer next month.
For me i feel its a better form of 'investment' in your car rather than exterior mod cos if we change car, ICE equips can be transferred easily, provided they still function properly.

just a quick check. Can a 4ch amp run both front comp and a sub?

thx!