Sealed in 2.5-3 cubes would be awesome for that sub. Your current enclosure is way too small and should be very peaky (and pretty loud) around 50-60hz with very little low end. I wouldn't port the sub at all unless you were going for about 5 cubic feet at 20hz. It's going to be pretty peaky ported unless you go big and tune low.
A 4th order with a front ported chamber of 1.5 cubic feet tuned to 45-50hz and a rear chamber of about 2-2.5 should sound very nice as well. That enclosure should get **** low too.