Thank you for everyone’s contribution in this discussion. It helped me make upgrade decisions to address my disappointment with the audio system in an otherwise fantastic vehicle. I have upgraded in stages to allow me to “stop” if I wanted but keep adding if I felt it necessary. I started with 2018 Limited, standard audio system, non HK, non RF. Sound quality 2.5/5 in my 55 year old rock/jazz oriented brain scale.
Stage One: Bought RF upgrade from Heuberger Subaru Parts on-line. Around $315 delivered. Rationale is that this is good starting point, designed for our vehicle and, most importantly, professional wire harness that allowed future upgrade path without tearing down dash more than once. Thank you to A-A-Ron for posting install breakdown. My install experience mirrors yours including removing that damn trim piece. Total install time was about two hours including front speaker installation. Sound improved to 3.25/5. New fronts reached deeper down but over-emphasis on mid bass at higher volumes sometimes obscured musical details. This is the audio quality I would have expected the car to come with from the factory to match the quality of the rest of the vehicle. I wanted more.
Stage Two: I decided to upgrade all speakers and add powered subwoofer. Again, thank you to all who posted! Vastly improved my pre-planning! Crutchfield and Amazon were my friends here. Install list - Kicker KSC3504 3.5” coaxial for dash, Polk Audio db6500 6.5” two way (only used woofer and crossover) for front doors, RF Prime R1675x2 coaxials in the back doors, NVX 8 inch powered subwoofer (QBUS8v2) for under driver seat. Metra speaker brackets and wiring harnesses from Crutchfield.
Install notes: The tweeter wiring is very limited as others have noted. Choice is to cut wires to extend or pry plastic connector off of stock tweeter after cutting or desoldering wire connectors to tweeter. I chose the latter and resoldered the connectors WITHOUT the crossover cap in the wire path. My 3.5 inch dash speakers receive the full audio signal. The speakers were drop in installation without modifications. Front and rear doors are pretty straightforward. May need to trim flanges and remove break tabs from adaptors foo fit your specific speakers. I used thick foam (weather seal type) between mated surfaces door to adaptor, adaptor to speaker. I used foam wrap and zip ties to ward off rattles and interference with window channels in doors. The subwoofer required the only homemade wiring harness. The speaker output wires from the RF amp are in the grey connector. I chose to use the front outputs for the subwoofer L/R high level inputs. White, White/Blk = Left, Grey, Grey/Blk = Right. Orange = signal wire. Wire taps with spade connectors formed connections - ugly but works. See attached photo.
I unbolted the rear console (under plastic covers in rearward section) to gently lift the console and pass my speaker and signal wiring to the driver side subwoofer space. Be careful to avoid interference with handbrake rod. I noted THREE red power and black ground thin wires going to the RF amp. My guess is that this is a way to meet power needs without thicker wiring. I left these alone and powered my sub by accessing the thick red/white power wire behind fuse cluster in driver lower dash. This wire is always live. I ran a fused 10 gauge power wire under driver side door scuff plate and under carpet to sub power. Grounded via short wire from threaded post under driver seat. I left enough wire length to swing subwoofer out rearward to make sound adjustments.
So now the soundstage is much fuller and bass can be felt as well as heard. Overall 4/5 on my scale and still tweaking. What I have come to learn is that as the quality of the components have improved, the limitations of the source material are becoming evident. Tested with a commercial CD and quality recording (e.g. Al DiMeola Orange and Blue), the sound comes to life. The same material streamed or listened to from compressed format REALLY affects the output. Without going down the rabbit hole of bit rates, etc, my point is that you may be wise to build your system to perform best with your most used source.
Final note (really). An audio-geeky thing to do is polarity test your speakers. For brevity, out of phase speakers (usually wired backward) can wash out sound and can be fixed easily. I used iOS app AudioTools from StudioSix Digital and a click track you can download from their site to check each speaker. Turns out my rear passenger speaker was wired backward (not sure why - connectors are all one-way). Quick visit inside the door and wire swap fixed this. Sounded better even from rear doors which admittedly have a minor contribution to overall sound, in my opinion.
Stage Three: Staying put for now…maybe larger amp to replace RF…