@jaytor --
It would be interesting to experiment with a pair of open baffle subs in a 2nd listening space and setup - they seem like a very capable design, not least as an overkill i.e.: über-sized option. 180Hz low-pass should be plenty enough to allow for a variety of design options above, like Danley’s Synergy horns or, like in your case, a line source.
Come to think it I listened to a pair of Quad ESL 63’s augmented I believe with a pair of open baffle fitted-to-the-Quad’s Gradient subs (dual 12" Peerless woofer per cab) back in the 90’s, and it was the best I ever heard the Quad’s. Indeed the pairing with the Gradient OB subs was excellent, and going by memory I’d favor that combo over the newer, more full range Quad iterations.
The TH’s are great for both music and movies, so yes, perhaps an OB solution wouldn’t offer the same versatile abilities here. Nonetheless they sound like an intriguing option.