For the record, I've some experience with the Supernova MKIV 15" sub. They are indeed excellent all around sub's, with plenty of output! I haven't run into a situation where I could bottom one out on their own, personally. As for music, not bad either!- and they blend very well.
All of this considering proper setup and tweaking, as usual.
Basically, I've gotten plenty good results with it.
Now, I would give the nod to something like the Paradigm Servo 15 for musical accuracy and speed. Infact, for $1500, I never have heard any sub that out-matches the Servo 15 for HT/music dubties for the money. Simply superb subwoofer, and a killer bargain!
However I've never heard a single subwoofer that I thought, on it's own, was the end-all-be-all of subs though. So, for the record, for the price of a single JL Fathom F113, you could buy TWO PARADIGM SERVO 15'S and have more dynamic headroom (two 15" subs of this caliber would allow you to run each sub with more efficiency, and less taxing on each), less distortion possible (properly placed, dual subs will cancel out any distortion differences between them, so you only hear their similarities), and simply more brute output than one single JL Sub! Of That, I garantee!!!
Plus you can run "stereo sub woofers" with two Servo's.
Of course, you wont' have that shiny black lacquer finish, then - lol
Just some thoughts. But, you can always pick up a JL and compare, and then you'll know