I was able to greatly improve the Bose system in my Boxster by upgrading / adding in new tweeters. It is still not fantastic but for $100 it was well worth it and livable for me personally. The real solution for excellent audio is to completely gut the system and replace it all with aftermarket components but that is beyond the scope of my Boxster that is worth $18k on a good day.

Make sure you are listening to a high quality audio source. I use Apple Music and have lossless audio turned on.

Parts List:
x2 BMW 2in Tweeters - 65139226357 $20-$40
x2 Dayton Audio ND20FB-4 $40
$10
x4 $15

Front Tweeters:
1. Remove the two front dash vents and remove the stock tweeters from the mounts. they are just clipped in place. Be carefully as the clips will be brittle.
2. Desolder and remove the stock crossover capacitor from the stock tweeter.
3. Solder in the BMW pigtail with the capacitor in series to the speaker
4. Slide in the new speaker into the bracket. Be careful as it will be a tight fit.
5. When reassembling, make sure the polarity is correct with the car. On the 981 the black stripe wire is ground, solid is positive

Rear Tweeters:
1. Pull back the carpet on the back firewall.
2. Remove the 1 T-30 Screw holding the speaker housing to the firewall.
3. Snap the tweeter into the housing.
4. Posi Tap the tweeter wires to the mid speaker wiring. On the 981 the black stripe wire is ground, solid is positive.
5. Reassemble the speaker assembly

EQ Settings:
+2 Bass
-3 Treble
0 Balance
0 Fader
No Surround
No AudioPilot
Yes Linear

The EQ is subject to individual taste but this should be a good baseline.