I'd reconsider getting a sub that doesn't at least have some pretense of being articulate. Your PSB speakers are pretty nice, and have a reputation for being precise, so getting a subwoofer that isn't might result in some audible integration issues. For example, that C1500K kit is a driver, amp and enclosure for $340. All those components for that price infers something has to give. Looking at the spec's depth is definitely one of them, but are dynamics another? Your room is not terribly large, at less that 1800 ft^3, so you may want to consider something smaller with better components.
You might check out the review of the PA-150 by Josh Ricci on data-bass.com, which he found to be an excellent sub for the same price with a 15" driver and a 250 watt amp. If Premier Acoustics can sell a good 15" sub at $350--and they have to build in profit for themselves AND for the reseller--no reason to suspect that Parts Express can't do the same given that Dayton is their house brand (same company) and thus have no middle man.