Well just heard a JL 12" w6 in an audio shop and wasn't impressed. It was being powered by a 2,000 watt amp so it was getting plenty of power. It got pretty loud, was in a special vented enclosure. BUT my old flat cone 10 k's from elemental designs killed it in the sq and spl category and they're in a sealed box. And I only paid a total of $240 for them, while a single w6 would have cost me $350 from that audio shop.
Someone I know that has a 13kv2 in a sealed enclosure seems pretty well matched in performance. The 13 kv2 that I heard didn't get quite as loud, but it was in a sealed box and sounded better than this w6 did.