OK, I went ahead and ordered the Phonocar 6.5" woofers. Just later, all your suggestions to mount a sub in a case in the trunk made me doubt. I tried to ask my installer whether I could change the woofers for a real sub. Obviously, he was not very happy about that and persuaded me to stick with my original choice.
And I don't regret it! The Phonocar woofers produce an astounding deep and natural bass. My installer did a real good job on the doors; the mounting hole had to be expanded and made more rigid, and he put a thick layer of some dampening material on the inside of the doors. Result: no vibration of the door at all.
My amp is now powering the front speakers (the Canton compo) at full range; the phonocar are powered lowpass (0-100 Hz). The empeg equalizer is set to almost flat.
I'm really pleased with the end result. If I would have gotten these speakers two weeks earlier, I bet I would have won the Best Sound competition at the Amersfoort meet, in spite of most other cars having real subs.
Of course this is not a boom-car. If you like to hear dance music at ultra-high volume then this is not your setup. Otherwise I can recommend it.
Another downside is that the installation was not very simple; in the end I paid more for the install than for the speakers.
Pim