Crossovers

Crossovers in headunits and most amplifiers have a 12db/oct. slope. Some high range amps have 24dB/oct. which is more than enough

6dB/oct is mostly used for tweeters.

Also, you can choose the cut-off freq. on the HU (there are 3-4 predefined values such as 50, 63, 80, 100, 125Hz). On my Pioneer DEH-7600MP I can choose between 50, 80 & 125Hz. Sub-out is really nice to have, especially if you want more flexibility from the system.