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The CDT CL61's are listed in the paperwork that comes with them as a 2 1/2"(7cm) mounting depth. The 5 1/4" are listed at 2 1/4"(6cm). The 4" have a 2"(5cm) depth. They are great speakers but do require a fair amount of power and some time imaging to really enjoy them fully....

maybe it's a self contained isobaric setup. Top sub wired normal, bottom wired out of phase. They'll push and pull in the same direction. That's all I could think of.