Here is a picture of everything tug away under the driver seat. The amp with all the mounting bracket and cover is too big to sit on the floor.

I am going to buy a impact driver to remove the screws and get it down to amp only. It should sit flat on the floor and keep amp cool.

 

One issue came up while testing. I only plug in the front mid but not tweeter. There was only very faint sound from the 2 front mids. I was worried and concern why the rear speakers are working but not front. Double checked all the wirings again, no error found. So, I decided to wire and connect tweeters anyway.when I turn the HU back on, all front speakers works. I am guessing the amp will detect and see if all speakers are present and working.
Now it sounds even better than the stock HU with Rockford forsgate speaker prime speaker upgrade. I got almost all the bass and more punch from the 2 door speakers. In my opinion, the amp has active cross over and separated the mid and tweeter sound very good.
I only connected my svc sub to see if it work. I understand this amp has dvc set up. So, the next project is to build a sub box to fit under the passenger seat.
Can it be better? Yes. But I am 95% happy with the $$$ I spend for this project so far.
Will post more pictures after I get the impact driver on Black Friday.

 

Ok. Got the impact driver on black Friday. Here are pictures of amp before and after.
Before.

After

tug away the wires and turn the plugs side front to hide the wires and prevent rear passenger kicking the connector.

I am going to build a speaker box for 8” sub under passenger seat. I may update that if it works out.
So far, the sound with dash tweeters, 6x9 front and 6.75 rear speakers are almost to my satisfaction. Just feel it is missing the low below 50hz. I assume it is because the amp/processor is sending it over to sub woofer I have not installed yet.
One thing interesting. I was testing all installed speakers. The rear speakers do no put out any bass at all. All mid and high only. I am speculating because the wrx sub is in the middle between the 2 rear speakers and and sub is filling in the low.

 

Hey Steven great post! Chris says you are welcome haha, I am indeed here.

I actually did a very similar thing in my crosstrek, adding a 2019 WRX amp with a 2019 radio. I'll share that post here: Model Year 2012 to 2017 - How to Swap in a 2019 WRX OEM...

It looks like we picked similar spots to tuck the amp away. I too used an impact driver to remove the metal bracket but left the plastic one on in case the amp got hot it would elevate it off of the floor carpet.

When I set this up I too was like "oh no, the front sound is too weak" but I realized the amp sends the highs to the tweeters and the lows to the front door speakers. Once I mounted everything it sounded pretty good to my ears.

Anyways great job and always nice to see someone sharing knowledge!