So I'm looking at upgrading the sound in my new WRX Wagon cause the bass leaves something to be desired. I don't plan on doing the full sub box in the back setup (I don't need or want that much bass) so I'm looking at two different options.

Option 1: Install Kenwood KSC-SW10 powered sub & amp under the passenger seat.

Option 2: Replace the factory speakers and power them with a 4 channel amp that matches their RMS power.

For this option I'm looking at the following components:
Speakers: JL Audio TR600-CXi (50 watt RMS)
Amp: Alpine MRP-F300 or Kenwood KAC-8404 (Are either of these decent for powering speakers)
Both of the amps will do 4x50 watts.

I've found the speakers and amp for about $300 and probably another $60 for the wiring kit and harness.

The Sub is only $125 on Amazon so the amp setup will cost around $235 more.

Option 1 is obviously easier then doing option 2, but I've read mixed reviews on the bang for the buck of the compact sub keeping the speakers stock.

If I did option 2, would this install method work:
I was thinking I would buy the metra forward and reverse wiring harness so I wouldn't have to run new speaker wire to all 4 doors. (Is the current run wire good enough to handle 50 watts?)

My plan would be to solder the harnesses together except for the speaker inputs and then connect the speaker lines coming from the HU side to the amp inputs and the lines that run to the doors to the amp outputs.

Is this a completely stupid idea? My hope is to do everything we I could easily put it back to stock if needed.

Also I have the premium WRX so it has the tweeters on the front doors. Will I need to worry about those being powered by the amp?