I have built more subs than we have members over the years for car audio and home applications. The various subs out there are all great, but for sheer sonics at reasonable levels, the subrosa sub is the favorite by far by bob. He said he though it was his best work. I agree, at low volumes, it is fabulous. There is so much going on in a bob sub . Limiters, thermal management for the voice coils, expanders, compressors, gates, etc. for the sub experience, i know personally that a x over either passive or active is not good enough unless the material has the sonics from the start ( low freq information in spades). Most older music is rolled off, and there fore there is circuits that help to this end to make the product have that sub rumble we all like.