I love my RiPol subs, which are essentially folded-baffle dipoles with a cardioid radiation pattern. The commercial versions are very expensive but my DIY version is affordable.
RiPol's are quality over quantity-- less efficient (85db) than boxed subs, so I always recommend a pair rather than one. Also, an outboard crossover is required, with a steep sloped low-pass <100Hz, and ideally a high-pass on the mains.
Here's a commercial version:
https://6moons.com/audioreview_articles/modalakustik/6-Moons Audio's review included this statement:
"For music-fist lovers who prioritize speed, articulation and enunciated clarity, it's the long-awaited messiah."
I share the same sentiment about mine (mine use the same woofers as the Modal Akustic sub).
A unique and amazing feature of the RiPol is that it lowers the woofer's nasty-loud fs by as much as 10Hz (-7Hz in mine), which means that it's unlikely to be excited or heard. Only a RiPol can do this.
Compared to conventional subs, a RiPol is a bit softer on the attack but much better controlled, and it's interaction with the room gives no hint of boominess. It plays profoundly low, and gives the clearest, tightest bass I've ever heard.
I freely share my RiPol sub CAD drawing and parts list with anyone who asks.
Here's a link to my write-up, and a photo:
http://jazzman-esl-page.blogspot.com/2011/01/ripole-subs-are-underway.html
