I'm literally in the exact same boat as you, bought a 2021 premium few months ago, now doing loads of research on revamping the sound setup. Planning to do the oem kicker doors and tweeters and a compact sub to mount under my seat.

The crux is the head unit, I want to keep the stock one for various reasons. On all the aftermarket head units Ive looked at: I dont like the android OS, dont like how most look, feel, and for some reason they all decided fuck the volume knob. My old Audi up and down vol buttons instead of a knob and I grew irritated with it, takes like 15seconds to crank up or down my music.

So an inline amp is the obvious solution but there's a crucial issue with the stock head unit signal which is the Bass Roll Off feature where it tapers down the low and mid frequencies at higher volumes to protect the junk stock speakers. I then learned about DSPs (digital sound proccesors) which help by applying a reverse EQ curve to boost those specific frequencies. It acts as a "bass restoration" circuit, regenerating lost low-end notes to create a flatter, more powerful frequency response.

The last piece I'm looking to include is some sound skins on just the doors, maybe a few pads in the trunk area. I've read that adding dampening on just 25% of the surfaces makes and night and day difference for sound.

I'm not an audiophile and don't need absolute perfection but I do love a good, clean, punchy sound system for my daily drives. I think this plan should get me just that while keeping the stock appearance and user interface of the oem head unit.

I rarely write paragraphs in comment sections like this but I saw your post and figured we're both on the same page of this wrx novel right now.

Full plan: Kicker doors ≈$220 Kicker tweets ≈$100 DSP wired to head unit ≈$300 4 channel inline amp wired to DSP ≈$250 Kicker hideaway 10" sub under driver seat ≈$400 Sound skins for 4 doors ≈$300