The Supercube 400 is the most like the MM8. Both have an 8" driver powered by a 1200 watt amp and vented by two 8" passive radiators. The Mirage is smaller as a 9" cube; the DefTech is more like a 12" cube and has a remote control. They have the same original list price, and a recent review in HT mag about the Def Tech indicates that it's a kick-ass sub for its size and price.
07-17-12: Tech_junkie
The mm8 seems to have the same concept as the DefTech super cubes?
However, bear in mind that since the MM8 is on closeout you can get a pair of them for quite a bit less than a single DefTech 4000 and at 9" on a side they MM8s will fit into tiny spaces that normally couldn't be considered for a sub.
Either choice will give you what you're looking for. It just comes down to budget and whether a 12" cube is small enough vs. one or two 9" cubes.