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 standard 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 maximum 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) for the 21" Cube and 16Hz up for the 21" Full. 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 under any circumstance.