To be fair, I wouldn't normally recommend mixing subs of greatly differing performance characteristics unless you know what you are doing, or unless you understood the behavior of the subs like the SVS subs, that they are protected from misbehavior. Sure it can be done, but most people don't have the time or know-how to make it all work. Dr. Earl Geddes made this the subject of his PhD thesis in acoustics, and Dr. Matthew Poes is his evil henchman. These men are insane fanatics who are willing to invest the time to gain the expertise in order to accomplish this. Most people, even audio enthusiasts, don't have the means or know-how to correctly measure their system, let alone place differing subs according to their optimal system-wide response. For those non-crazy people, a group that I sadly do not belong to, they should probably just get subs of similar capabilities and place them in room according to certain rules of thumb, like Harman's or Geddes' typical placement recommendations.