Others experience re: subs and Magmepan 20

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