There are a ton of good sub options out there these days, but a sub with a lot of low-frequency extension (which appears to be what you are looking for in this new sub) is either going to come in a large cabinet or in a smaller cabinet at a (potentially much) higher price. Both the SuperCube and the SuperNova tend to land in the latter category. The price on that Supernova is pretty impressive (about 50% of the list price, according to Earthquake's site), although I don't know what warranty might be offered with it.

I tend to look toward companies like Outlaw, HSU Research (who was involved in the Outlaw subs), or AV123 for good subwoofers at a good value. For example that borrows from our forum host company's product line, a LFM-1 EX that costs $600 will get down to 22Hz, which is on par with the 23Hz extension of the MKIV-12, and can be configured for maximum extension mode (plugging one port) to reach down to 16Hz. It's not an apples-to-apples comparison in a lot of ways (different enclosure designs, for example), but it gives you some idea of what's available. Heck, the LFM-1 Plus (with a $500 price tag) will do down to 18Hz when in max extension mode.

Some of the key factors in picking a sub are room size, available space, listening goals (how much bass do you need), and budget. If the room size is medium to large (say 3000 to 4000 cubic feet) and you have the room for a sub the size of an LFM-1 or similar ported design, you can get something that will go to or below 20Hz without having to pay as much as you would for a small sub like the SuperCube or Supernova.