So basically M100 has more (bloated) subbass, at least that's what I interpret from reading all the comments and having understandment regarding bass characteristics and how midbass vs subbass sounds and is percieved.
I often see people comparing the M100 bass against Sennheiser HD25-II and M80 as they have vastly different kind of bass, the HD25 and M80 have a more punchy bass due to more midbass focus, they both don't have overly large bass quantities, reasonably modest boost but they pack a lot of punch for the bass quantity they pack, at least the HD25-II I was VERY surprised about actually how strong of a midbass punch they pack for the amount bass they measure, I've demoed far greater bass boosted headphones with noticeably less punch in the bass (due to greater subbass emphasis). Midbass = fast decay (solid "hit", compare hitting a hammer against a table kind of experience or whatever), subbass = slow decay (Home theater "rumbliness" bass, softer impact).
It's therefore the bass quantity can be hard to measure subjectively as the subbass and midbass sounds so different. Often I see people hating midbass around here and associate it with something that bleeds and muddens up the mids. But there is various quality midbass output. Good quality midbass output is best explained as standing near a drumset, you feel a physical impact as the drumstick hits the drum that is very short but has a strong impact to it, the same kind of impact is very well percieved on for example HD25-II despite not being OVERLY bassy, it's just they offer very good quality midbass emphasis. Anyway this is most certainly the reason some people think M80 is bassier than M100, they actually like good quality midbass output and would like to hear more midbass skewed output on M100 but the subbass range is stealing the show and makes it more deep rumbly / soft / diffuze / bloated / (insert typically used adjectives here)
+! My personal preferences have always leaned towards sub-bass. My appreciation is for the kind of bass that I can feel (not only in headphones but car and home audio). I like that the M100 are efficient enough to reproduce those low frequencies with a strong feel without sounding distorted or over the top.