I know this wasn't mentioned by the OP, so I hope you don't think I'm going OT. I'd be remiss if I didn't suggest Funk Audio. Bar none, some of the finest subs for the money. The TSAD18v1 driver is an absolute monster. The prices are great, and while the 18.0C would be a fabulous fit X2 you can go for the slightly less pretty version of the 18.0 which is called the 18.0SE.
With those subs you don't have to choose between musicality and low end. They can x over insanely high, and have phenomenal output below 20hz. Check out data-bass for testing results, but as far as commercial subs go, it's right up on the Sub2's wheelhouse. 2.4kw amp, 110 or 220v option, devastating output. Those measurements are 2m groundplane open field. In room performance sees better performance yet.
I was where you are. A friend of mine picked up an 18.0C, stunning performer for the money, devastating even. But the 18.0 or 18.0 SE is far beyond that. Nathan seems to offer discounts for second subs as I recall. He's a great guy and worth a call...even if you want to Co firm it's NOT the sub for you. If your batcrap crazy, the 18.2 while not officially tested by Ricci yet, is another level all together. He might even have crazier designs. He does custom anything, but has the house models too.
Not trying to oversell, and certainly not downplaying the other options, they are all amazing. But frankly the 18.0's go where rythmiks can't, and for a sealed design, if you want sub2 performance for a fraction of the price, then take a look. I love the Sub2 BTW. But by design, you couldn't make a sub any more expensively.
Anyhow hope that helps. I'm sorry if this is a repeat, I didn't have time to read the whole thread...so apologies if you already pulled the trigger.