a lot of people really liked the older xxx in ported enclosures. IIRC the Q was a tad high for that though and it did always seem a bit boomy to me.. I had a AA Avalanche which was a similar sub and it had the same issue.. You could be happier with a lower Q sub ported, that would cut the boom out.. Anyways if you have the room if you wanted to go sealed you could look into lots of options.
You can often find the FI Q's for sale, if you can find the high Q versions, they are good in sealed boxes and match up very well to a XXX, I'd go for at least 2 15's sealed, 2 12's might not give you much more output.
If you want to go new, a set of dayton HO 15's would do great sealed.. They dont' have the xmax of a FI Q, but they will play upper bass better, def a MUCH different sound than the XXX and even the Q.. They can still get low, but they will play upper bass well, "punchy" if you will. Might like this for EDM and rock, as they have a lot of upper bass.
Other options would be SSA ICONS or XCONS, depending what you can find, often Xcons are good values used, it seems like to me..