There is a review of the SuperCube 4000 at Home Theater's
website and latest print issue.
They gave it a decent review: "Considering all the extremely useful features the SuperCube 4000 has combined with its small size, exceptional musicality, and distortion-damping muscle, I think it’s a spectacular bargain for under $800. Definitive Technology has always had a habit of bringing together the right amount of performance with a knack for the perfect contemporary design in order to create affordable speakers that look as good as they sound (and vice versa). With the SuperCube 4000, the company’s gone one step better and added a package of features that make this subwoofer a true standout.
The -3dB point was 26Hz and the -6dB point was 24Hz... I would expect that the SC6000 should go a tad lower, probably 24/22Hz at the same reference points which isn't too bad (my Polk PSW650s with dual 10" active drivers only goes down to 32Hz). They did say that the DSP prevents the sub from distorting when overdriven. They also mentioned that the sub bass was very tight and good with music. The frequency response of the SC4000 is shown below, looks decent for the sub overall size. They admit that the sub small size limits the ability to "pressurize" the room or shake the walls.
The only thing I have against Def Tech is the stupid sock design they have on most of their speakers. It damages easily and makes the sub look cheap.
As a side note, I am currently using a dual Polk PSW650 subwoofer system in my room. It is very tight but the bottom end is weak, going down only to 30-35Hz. They have no problem in delivering the volume, but it just can't "shake" the room. I also have a Sunfire True Subwoofer Mark II. This sub can deliver a LOT of bass. The whole house rocks from the bass when the movies delivers any sort of explosion. But the bass is just not tight. It sounds blubbery or loose. That's why I reverted back to the twin Polk subs. I do have a SC6000 on order and hope to get it in the next couple of weeks. When I do, I will post my feelings on the sc6000.