you have a high output box, but say its too big, you probably have to go bigger for more SPL, or to increase the efficiency of the enclosure.
If you have that amp driving the sub its seeing something like 260 watts, more power is in order here.
I was gonna recomend sealed but you would need 1k to get it loud in 1.5 cubes sealed.
maybe sell the box that you have it in, its not gonna get too much louder ported in another design but someone out there might have a more efficent ported box design.???