So I just pulled out my M, 1 ohm adapter and the PF IX, and went down into the garden to kick back with a coffee and cigar and give it a good listen.
First of all: I heard zero evidence of any compatibility issues with the music that I wanted to listen to. I later tested a song that has some opening bass notes that always caused a problem and they still did. 1 ohm was not enough to fix the issue. It is strange though; it is just a few notes at the beginning of that song (The Little Beggar Girl by Richard and Linda Thompson; the perfect test song for this issue) and then everything is fine. Hopefully the adapter Calyx provides will have higher impedance. Again, there were zero issues with the other music I listened to.
I listened to some Baroque and it was perfect. Perfect, I say! I will go out on a limb and posit that harpsichord never sounds better. A private church in my head... unbelievable. String ensembles were equally compelling.
I then put on Jazz, the Dave Brubeck Quartet. Blue Rondo A La Turk was excellent but the M has a slightly mellow presentation and I was thinking that I preferred the more dynamic sound of the Wyrd + Modi + GS-X mk2 that I was listening to the same song on with the PF IX yesterday. I little more sparkle and definition on the cymbals in particular. Later when I was listening to Take 5, a song with a more "clubby" sound, I could find no fault at all. Exquisite. A private Jazz club in my head. (Edit: Regarding "no fault at all", I realise that I am so used to the PF IX that I just take the paucity of sub-bass for granted. The PF IX will always be the PF IX...)
I put on some acoustic guitar plus vocal material (the bonus disc from Jewel's Sweet and Wild.) The M + PF IX does that kind of music well. The smooth presentation of the M tames the treble edge of Jewel's vocals. Very nice.
Then for the heck of it, I put on Joan Osborne's One Of Us. It was... pretty obnoxious. No fault of the M; the Piano Forte IX does not do rock, even light rock like that. Mind you, Joan Armatrading's All The Way From America was better. I didn't like the hard drum slaps at all but the vocals were great. With Rock/Pop, you are best to keep things acoustic and mellow with the PF IX.
So I think that the M and PF IX is a great combination when used with the kind of music that suits the PF IX. Once the official adapter comes out, I expect it to get some fair amount of use. (There is still the zero isolation fact of the 160Xs; quiet environments only.)
As for the impedance issue, it is so rare that I really have to look for it. I have decided that I don't care very much with the PF IX since the music that the PF IX does well almost never causes the problem. Still I look forward to the adapter.