Let me do some expectation managment. Given the size of your room a single sealed sub probably won't cut it - especially not one in a small enclosure. Off the top of my head I'd say you'd need at least two 15" subs and ported might work better here as you won't be getting much room gain. Also for $300 you aren't going to match a Fathom.
If you are hung up on sealed and can stretch your budget I'd buy
two Tempest-X sub drivers ($350) to use with that amp and put each in at least a 3 cubic foot sealed enclosure with a lot of polyfill. These subs use the same frames as the 15" Dayton Titanics so they should be a drop-in fit for the PE cabinets.
I am a bit hesitant to make recommendations though as I can't find any specs for the impedence of Velodyne's passive subs. I would assume they are 4 ohm but I can't be sure and you know what they say about assumptions.