Actually I'd say its the other way round: the sound of an electric guitar (or any instrument thats not strictly acoustic) is always an interpretation done by electronics and therefore influenced by how they postprocess sound. An acoustic instrument (or voice for that matter) can be heard in reality so to say, it *does* have a definitive master version (though it is often used as an instrument in the sense that the voice is postprocessed as well, be it the choice of microphone or the effects applied).
As the saying goes: "nobody has ever heard a synthesizer" -- but only whats coming out of the speaker at last. So I think a realistic reproduction of voices is one of the most complicated feats to accomplish in audio: we're all used to hear voices (no, not *those* voices in your head ;-) and are therefore very well-versed in differentiation of natural vs. artificial sounding voices.
And No: the M-100 does not present natural sounding voices. Its a fun headphone with respectable sound quality, therefore the question to ask is not "how realistic are they?" but more "do I like their coloration?" So, while I can't comment on the M50, which I've never heard, I think, the Focal Spirit Pro or the HP50 are much more natural sounding than the M100. Thats what the first two are built for, and they do it quite well. The latter is for fun, enjoying music -- if you like its take on music that is.
With electric guitars the reference is how the guitar amplifier sounds in person in the room coming from the cabinet. Everything between the speaker cone in the guitar cabinet and the AD converters affects the sound, but in the end the goal is to capture what it sounds like in person sitting right in front of it.
The second point of reference is how the guitar sounded coming from the studio monitors which is where you first hear the result of everything in the chain that is trying to capture the sound of the amp in person.
I feel like the M50s just can't reproduce really complex sounds compared to the M100. It's missing a lot of detail and transients and sounds ugly overall.
I guess what I'm trying to say is that the M50s are like a polyester sweater and the M100s are like a pair of silk underwear.