subwoofers arent "stable" at certain loads. They can be wired to certain loads, all of which are fine for the sub. On the other hand, amps are only stable at certain loadsd. Some only can go down to 2 ohms, and others can run at .35.
With the type x, if youre running just one, it can be wired to 2 ohms or 8 ohms. Which amp do you plan on running?