The one chord that always trips me up is the 4-string F chord (truncated barr version with 4 strings). If I play it with a blues grip to mute the low strings, I lose power in the first finger and sometimes this results in choking the high E. So I'll sometimes switch to a classic grip. It sure frets easily with that grip, but then the lower strings are tougher to mute (I can get the 5th with the left hand ring finger), and it's also harder to switch out of that grip into a blues grip on the C. If you have any recommendation I am all ears.[
I'm virtually purely an acoustic player and haven't played anything but my 3 Guilds in the last 20 years, anything else would have been due to borrowing a buddy's axe for a tune or 2.
I've developed a better ability to only hit the strings I want even when aggressive strumming, and use a lot of right hand palm muting that might handle the lower string F muting you mention.
To tell the truth I'm not sure exactly what you mean by a "Blues grip on the C", (I suspect it's got more relevance to playing an electric if it's what I think it might be, "hooking the thumb over") but perhaps you're talking about a barred C as opposed to the "cowboy chord' voicing?
Sometimes if I run into a chord change problem like that, that's what I'll do, use a different fingering of the chord.
Also opens up new scale possibilities.
It's like the one thing I have never been able to figure out, and I think it has to do a lot with the nut.
I do remember that when I got the D25 it was like a revelation, all of a sudden stuff that used to be a little tough was easier.
The whole set-up really was "Made to be played".
It's why when testing a guitar the first chord I grip is an F. If the nut is messed up at all the F chord will make that obvious.
On this M-20, the nut slots where really good on the lower 4 strings, but the upper 2 had too high a nut, and sure enough that F chord was difficult. On my GF's Martin, straight from the factory on a cheap $350 model, the F chord is a breeze.
It does sound like simply getting those E and A slots a few thousandths deeper might help. Assuming you decide to keep it...
It'd also help to actually have had a new M20 in hand to see how it felt, but the last time I was in my favorite store and hoped to actually find that out, they hadn't brought any in...