The problem is lack of depth within the door cavity to allow a good quality speaker to be fitted .There is a 1/2" deep spacer which was supplied by porsche for some of the later models which allows you to fit a 6x4" with a deeper cone.It fits over the original hole in the door card and the deeper speakers fix to the standard holes. You then fix the grilles over that. I saw it on one of the forums recently somewhere.
Heres a useful link to 944 ICE
http://www.connact.com/~kgross/FAQ/944faqst.htmlthe relevant bit is copied below from the connact.com site
A second style of 4x6 mounting was used in some later cars. This is a two-piece assembly consisting of a thick plastic frame and a snap-on grille. The grille area is made of metal with a plastic surround. Using this grille, it is possible to mount speakers as deep as about 2". For example, an Audax VE4X6F0 mid/woofer, which is 52 mm deep, barely fits.
The parts list for the later grille assembly is:
944-555-149-00-01C Grille (2 needed)
944-555-159-00 Frame (2 needed)
477-035-375 Bushing (8 needed)
043-938-3 3.9x22 lens-head sheet metal screw (8 needed)
011-522-7 Washer (8 needed)
Note that the bushings are important to space the frame out far enough from the door panel to allow the grilles to snap on. Also note that the replacement speakers MUST be the European oval shape – not rectangular. The replacement speakers must also have a thin steel mounting frame like the OEM speaker - not a thick plastic one.
The least expensive approach to mounting a deeper 4x6" speaker may be to make a simple spacer out of, say, 1/2" MDF and use the flat waffle grilles. Spacing the speaker much further out than this is not possible as part of the grille is covered by the dash when the door is closed. (924 and early 944 have more clearance than later 944 and 968.)
If you are installing coaxial speakers, also make sure the tweeter does not protrude too far. For example, certain Infinity speakers have this problem. The JBL GTO-462d does not.
Last, many people have replaced the oval 4x6" speakers with ‘4x6 plate’ or round coaxial speakers. All other things being equal, which they aren’t, a 4x6 woofer should be more efficient at producing bass than a smaller round speaker. So if you do not plan to use a subwoofer, you may wish to stay with the oval form factor.