Magnapan's are dipoles, so you might want to consider a pair of dipolar subs.
I'm very fond of a very special type of dipolar sub called a "Ridtahler dipole" or "Ripol" for short. It's a very compact OB with its baffle enfolded around a pair of opposing woofers in push/push configuration. It's off axis output is nulled so it tends not to excite room resonances that can make the bass sound sluggish and boomy.
The most interesting characteristic of a Ripol is its effect on the woofers' resonance. Whereas a woofer that resonates at 25Hz in free air would resonate at around 47Hz in a 150 liter sealed box, but the same woofer would resonate way down at 18Hz in a Ripol. This has a profoundly positive effect on bass quality, because its then unlikely that the resonance would ever become exited, and even if it were, it would be infrasonic where you wouldn't actually hear it as a tone.
Ripol's are not for everyone because they are rather inefficient (a pair is better than one), and they don't don't hit you in the chest like a sledge hammer, but their are simply the cleanest, purest sounding, and most non-boomy subs I've ever heard.
Modal Akustik in Germany markets a very nice Ripol sub.
I build my own, and I am happy to share my drawing and parts list with anyone who asks.
Mine DIY Ripol subs are shown below:
