[Elmer]

Here's what I did last year:

I recently replaced the entire factory Bose sound system for higher quality components. The Porsche/Becker/MOST system really limits the available options. Like you, I wanted Apple CarPlay and BT. There are lots of head units out there, but if you're wanting CarPlay, do your research, read reviews of head units to find a reliable, easy to use model. The factory amp is a five channel, under powered, MOST-only component. Amps are relatively inexpensive. You can interface with the factory speaker wiring and there are several websites that have the wiring diagrams. I think theRennTech site has posted several. The dash speakers, for the Hifi and Bose are component, 1 in tweeters and 3 in midrange drivers. The Car Audio Innovations website (cai-store.com) has adapters to mount 4 in coaxial speakers in that space. That's a great upgrade, and can be relatively inexpensive. The door speakers are 5 1/2 in.

If you have the Bose system, the wiring from amp to speakers is a bit wonky so that Porsche/Bose could use a cheap amp to get acceptable sound volume. The speaker drivers are fairly low quality, but can be made to sound good with a cleaner amp w/ a bit more power. But, they wired the door speakers to 2 ohm load and the rear "subwoofer" enclosure to a 1 or 2 ohm load.

Wanting Apple CarPlay in a double DIN configuration, I replaced the CDR 23 head unit and amp in my 03 Boxster S with a Kenwood HU and amp. That solved the MOST nonsense. The installer wired into the existing speakers. Sounded much better. CarPlay is wonderful. BT is handy. But, the Bose drivers are flat, bass is muddy, dash speakers not clear but very biased towards voice and "bright".

Next step, replace all speaker drivers. Rod at CAI Store helped with great advice and a few products. Some research online helped with other aspects. I ended up the CAI Store dash mount Hertz 4 in coax speakers. For the doors, I mounted Hertz 5 1/2 in woofers from the component set and put the tweeters in the rear deck speaker enclosure where the Bose tweeters were previously. The crossovers are mounted in the trunk next to the amp. The woofers in the rear box were replaced with Morel 5 1/2 in woofers. (I stumbled across this build plan on the DIY Audio website)

How does it sound? Wow!!! It's an amazing transformation. Certainly, the signal processing capabilities of a modern head unit help tremendously (Kenwood DDX 8905S and Kenwood XR 901-5 amp). The speakers are very clear and driven by an amp that has enough power to drive them efficiently. The rear deck subwoofer has "deep" tight bass. To say that I'm pleased would be an understatement.

I don't know if there's an easy way to get where you are wanting to go. The MOST bus is severely constricting to adding modern audio.