(enter Prowler with his normal eXcelon recommendation...)
The XXV-01D has the option of choosing a 12dB/octave, 18dB/octave, or 24dB/octave slope for the crossover settings for front, for rear, and for the subwoofer preout each setting being independent of the others. (ergo you could set a 12dB slope for the front, an 18dB slope for the rear, and a 24dB slope for the sub if you choose)
I will assume other top-end eXcelon units have similar features such as the KDC-X889, KDC-X890, KDC-X990 but I cannot say for certain having never had my hands on any of them.