These enclosures have low tunings optimized for home theater just like MartySubs. With a good 21" driver, they will provide exceptional midbass as well as a deep, powerful, low-end for movies (kind of like getting a MartySub and MBM all in one unit).
These enclosures also make exceptional music subs for folks who enjoy pipe organ (with 16Hz capabilities) and/or folks who want to capture the very lowest low-frequency-effects in some digital music.
As a tradeoff for their lower tuning, these enclosures have about 6dB less output in the 25-35Hz region as compared with the 21-MBM. So for music only systems that are targeting rock concert SPL (130dB+) from one or two enclosures, we recommend the 21-MBM.
With four enclosures, maximum output capability exceeds 130dB from 18Hz up (before adding in any room gain). If you have a budget for four enclosures, you may as well go for the low-tuned HT enclosures since it is unlikely that output in excess of 130dB is required.
Note: We are developing a taller version of the 21-inch Cube that will have a slightly lower tuning that, with four cabs, should exceed 130dB from 16Hz up (before adding in any room gain) for folks with room for modestly taller enclosures (height will be less than a Full Marty, but specifics will be provided when design and testing are complete).