Others experience re: subs and Magmepan 20

akg_ca

There is a kind of snobbery that exists in the world of high-end audio ...  ... the crossover that comes with almost every subwoofer on the market will cause more damage to your signal than can be overcome by these benefits ... I don’t care how big your main speakers are, they’re no match for a good subwoofer in the bass. A really good subwoofer can run rings around the best floorstanding speakers ... Vandersteen has the perfect solution ... there is only one subwoofer in existence capable of integrating seamlessly into a high-end music system, allowing you to reap all of the benefits of having a subwoofer, with none of the drawbacks. And the Vandersteen 2Wq is the one ...  the Vandersteen 2Wq subwoofer is a seamless extension of any system ... the Vandersteen 2Wq will exceed your expectations

There is no one subwoofer, there is no one component, that is perfect in every system. The world of audio just isn't that simple.

My friend works for one of the top 5 orchestras in the US. He listens to live classical music onstage, backstage and from the audience perspective almost every day. He owns 20.7s and a pair of REL subs which are fed line level signal and run <80hz. It's a big improvement in his room vs. the 20.7s alone. He wasn't happy before getting the subs. Not that it matters much in the context of the question but FWIW, he runs Rogue M180 monoblocks, and uses both vinyl and digital as sources. 
OTOH, I run Sound Lab A3s in a medium sized room(23x17x8") and can't imagine that subs would make a big difference in my own system. 
BTW, for use with electrostatics Rhythmic is another manufacturer that I've heard a few folks say is a potentially great match. Cheers,
Spencer

I used to use Janis Subs back the in late ’70s & early '80s. The crossover was 100 Hz. Is yours lower than that? 100 Hz is way too high for those Maggies.

I use two Rythmik F12G with my Magnepan 3.7i's and have the crossover set at 60.  The coherence is pretty good, but there's a slight dip at around 70hz.

+1 for Akg. I think the Vandersteen subs coupled with the crossovers will be the best possible combination for your Maggies.
But, since you were asking not for recommendations, but opinions regarding subs, I would have to say that whether ' they feel they got substantial benefit' would depend upon their integration as well as what subs they are using.
Though you can get the low frequencies, you have to remember that those frequencies must be reproduced in a balanced level with your main speakers. Once they stray significantly from a flat response (as well as phase), you loose all semblance of realism. I am sure others can explain this better, but I tried.
B