There are lots that use 12db in both directions. Some that use 24dB low pass and 12dB high pass (THX certified) that rely on the speaker having a 12dB/octave roll off starting at 80hz. Some like the B&K that allow you to choose 12dB or 24dB for the low pass and 6dB or 12dB for the high pass. Even the Outlaw ICBM is only 2nd order except for a special low pass which can be 6th order. But why would you want to mix 2nd and 6th order filters? Is a 2nd order crossover steep enough to prevent too much above 100hz output from the sub. Is there some problem with using a 4th order crossover for bass management?