For tweeters I use modified Aurasound NSW2 drivers (cone is slit and coated with damping spray and rear chamber is staffed with foam) attached to small horns. they cover ~1khz-15khz
For midbass I use modified Dayton Audio PA165-8.
Subwoofer is 15in driver from diycable.com (very nice sub)
Foam acts as lowpass filter and provides smooth high frequencies roll off.
I use a single inductor for woofer's crossover. if you use metal drivers, notch filter has to be implemented, foam won't filter out metallic resonances.
This is frequency response(red) at driver's sitting position. It looks like high frequencies are recessed too much, in reality tonal balance is just right.
nearfield measurements of woofer and tweeter (system is playing pink noise)
frequency response on tweeter axis, mic is 8in away
So, my conclusion is that car system needs to be calibrated on tweeter axis, no matter where tweeter is installed. If I tried to achieve flat frequency response at drivers position, sound would be very bright and unnatural.
Of course there are still some problems in my install, particularly that hump at ~300hz