Rythmik subwoofer

Is there no one out there that uses Rythmik subs with Magnepan speakers?

I think you'll find a number of threads on the Rythmik products- I have a few of their subs but don't use them with Maggies. If @bdp24 reads this, he may help b/c he is familiar with their use in conjunction with planars. They make an open baffle type as a kit which requires you to provide the frame--something that is spec'd out for DIY or for a competent carpenter to help you assemble. 
The company itself is good, as are their finished products.
I can tell you based on my experience that matching dynamic woofers with electrostats or horns is tricky, part of it depends on placement, X-over frequency, how many subs you employ and your willingness to do measurements and make adjustments. If the L12 is ported, it is not going to be comparable to the sealed box in sound. They also offer a choice of drivers, with the paper cone (GR?) version being more desirable for music. Hope that helps. Brian at Rythmik is a pretty honest guy and easy to deal w/. 

The Rythmik dipole subs will be a good match for use with dipoles like the Magnepans. But do not forget to use dsp room eq such as the Antimode 8033 as well.