Any response measurements of that 12" with the ports open vs closed? I can't imagine it's doing it's best at all three options... Personally, it doesn't matter too much since I don't do HT stuff, so for music, a smaller sealed sub is ideal. Given that my speakers are running on about 5W, 500W 12" subs are a little bit overkill.
Honestly though, for the price, the SVS PB2000 is a better choice in my mind at that size anyway. All else equal, Monoprice's customer service isn't anywhere near as good as SVS' is. And the reality of it is, unless these are held to higher standards than their headphones have been, Monoprice's build quality is a bit lacking.