Getting to and/or removing the tweeters has been discussed here...
http://www.planetporsche.net/cayman-...-tweeters.html. But I've been an incorrigible audio tweak for about 35 years, so I just had to see what the tweeters look like, how difficult it might be to replace them, and especially if the 'crossover network' is mounted at the tweeter. This is the case--the crossover is merely a 4.7uF/50VDC inexpensive nonpolar electrolytic* capacitor, and it's mounted directly on the tweeter's back.
The tweeter itself is a 3/4" cloth dome, better than I thought it would be.
Altho I've not had the feeling in my CS of the music coming from my ankles or shins, I still would like to have a little more of the music coming from in front of me, so part of my quest was to lower the filter point to get a little more upper-midrange coming from the tweeters.
WARNING--if you listen at high levels and/or with the treble boosted, do not increase the value of this 4.7uF cap unless you want to risk frying your tweeters. I don't listen at high levels, so I replaced the 4.7uF cap with a 10uF cap, lowering the crossover frequency a little more than one octave. Also I wanted to increase the quality of the treble by increasing the quality of the filter cap; I used BlackGate caps that I had in stock. Unfortunately they're no longer in production or available that I know of, so maybe some fellow audiofool can recommend a different high-quality cap. Here are some pics.
Whether you keep the same value or increase it, start by lifting the cap from the tweeter.
Unsolder the cap one lead at a time. Since the tweeter's leads are VERY small and probably fragile, I used an alligatorclip to heat-sink the strip the cap is soldered to. I strongly recommend
against using a soldering gun for this work; a low-wattage soldering pencil is MUCH safer. If you'll be increasing the value of the cap, remove the little ledge that locates one side of the cap. I just twisted it off with needle-nose pliers.
Place the new cap in the now-wider slot and solder the leads, one at a time, snipping the excess after the solder cools.

If you're concerned about the cap falling off the tweeter, use a dab of siliconseal or hotglue to anchor it; I did not.
I have listened just a little since finishing this, initial results are positive. More later.
* the worst-sounding capacitor type bar none. But they're small and cheap.