Ahh... But the specs -
NIC's of just over 30 - Pretty much the same as a insulated drywall over wood studs wall. And notice that the ratings are tested at *over* 100Hz. I wonder why...
Because everything below that passes right through as if it weren't there at all. That's why. You can't try to contain low end in a space that small - You have to let it go somewhere.
And the WEIGHT - 650 pounds for that tiny little box - That's a lot of mass in a very small space. IF that mass is used correctly, it can absorb a good amount of lower midrange. A 4x4x8 closet, drywall, studs, insulation and all would only weight in at around 300 pounds.
So, the thing probably works fairly well for what it is - But it's made for isolation - Not for superior sound quality.
And as I mentioned somewhere (maybe not this thread, but somewhere around here) if you ripped out all of your drywall, filled it with OC703 or 8#/cu.ft. rockwool and used pegboard for the outer walls, you'd probably end up with similar effectiveness. Still not ideal though.