Others experience re: subs and Magmepan 20

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

"....My question is .... but rather other folks experiences who have tried this and do they feel they got substantial benefit from an extra half octave added to (almost) full range speakers to justify the expense, hassle and real estate taken up in the music room..."

In short....you betcha....a big-time audio performance improvement BUT only if done "right" as highlighted in the reposted article.

I have seen and heard too many subwoofers introduced that simply bloat , muddy, chuff and destroy the audio reproduction simply because they were a crap and  crummy match to the 2-channel speakers primarily because they were indeed cheaply built kit .... full stop.

To CLEEDS
"....
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...."

I never said it was the final single bests-all-comers solution ..... it wasn’t my article ...just a link to a third party article with a quality background read highlighting on the strengths and warts of ALL subwoofers, ( note the article title ) that also happened to include that writers suggested fave in closing that you can reject if you want to.

I know, you know, we ALL know that there is no one-size fits all to perfection choice, to quote your metaphor.

The Vandy is a very very fine contender AT ITS PRICEPOINT in a large arena of pretenders ,

BUt

I would gladly upgrade further to, say .... an ATC C-1 subwoofer that starts out north of $10,000 and goes  up,(if I could afford it) as just one simple example.