Can I join in on the party?!
A buddy of mine has a SQ/SPL set up in his Dodge ext king cab. 2 15's ported to 36Hz for SPL and a single sealed 12" JL sub for SQ. A master switch can be used to listen to either. Paired with an assortment of Hybrid Audio speakers mids/tweets, it's pretty loud but not blow the doors off loud. Lastly, no matter what volume it's at, it sounds good overall.
Case in point, the fact that sealed sounds better than ported isn't true. Some factors that play into a good sounding box is prob the most important is a good design, then the tuning and bracing to prevent any standing waves inside.
Take Jeffs suggestion and run with it. You're not gonna get any louder with the same subs in another sealed box on 500 watts. You want louder, go ported. Or go home. Take ya pick.
Lastly, I'd like to know what these sealed boxed subs you have heard that were insanely loud to you. Pics would be great. A link works to.