Are you actually pushing the current subwoofer to (or beyond) its limits in your workshop...seriously? 4,000 cubes or not, you should be able to get pretty good (read: sensible workshop output) response out of the driver in a 5 cube cabinet. But perhaps my expectations for "shop bass" are grossly distorted. Even so, will another 2-6 dbs really change much? To even get to 5-6 dbs of additional output, you're talking about time and cost FAR exceeding that of adding a port.
Further, I didn't realize the Dayton HO was "low end". Because it's only ~$250 from a online warehouse supplier or because it's not a TC Sounds Ultra or UXL 18? I'd be willing to bet it's actually closer to an "8" on the 1-10 18" subwoofer performance scale when you take an honest look at the full scope of 18" drivers available.
Actual phone call (see pic to left):
Tech (responding to laughter): "I'm sorry sir, did I miss something?"
Me: "Yeah, a case of Diet Mountain Dew walking across my living room."