i have a 13w7 and love JL. the x.x.x. 18 has great spl and good sound quality. it matches up with a w7 of the same size, but i think JL has a tiny edge in SQ
I personally like the SQ of the X.X.X. better than the W7(for the equivalent size sub), but it's always subjective. The 18" X.X.X. has good sound quality for the sub that it is, but not the greatest being an 18, cone resonance and sheer mass come into play. I'd do a 15 X.X.X. to have a great all around sub, the 18 is more SPL inclined. Can't really think of any subs to compare it to, it has good SQ, just slower to respond after certain higher frequencies than smaller subs, and deeper overall performance. Not good at midbass transition, you'd need a really good component set with great midbass, or dedicated midbass drivers.