I guess you meant 1000 @ 1 ohm, 650 watts it´s VERY small power, specially to that "beefy" sub wich doesn´t look bad at all, ok i´m gonna try to explain as easy as i can, small box gives you more cone control, you need good power so the sub can get really loud, big box need less power but also, they tend to get louder on low frequencies than a small box; Said that, with the power you have, i would build a big box so you can be kind of loud and get the lows nice also.
I ain´t suggesting you a big ass box, but, i would build something like 60 liters, but that´s just me, and 31 SQ means 31" of port area , for example, 10"W x 3.1"H , 6.2"H x 5.5"W , etc etc.
So you're suggesting I should make a square port? cause you're talking about width and height? How about the depth? I'm sorry, I really have no clue how to tune a port correctly and every calculator I use gives me other results.
I know my amp isn't that amazing at all but I don't have money for a better one yet. They said a good amp for my sub is a 1000 watt RMS @ 4 ohm.
By the way, you're system is sick man!