I think you should post to the bass-heads thread and ask there for help, my listening tastes are not bassheavy.
Well you can't control the volume of the ZO2.3 in high gain so you simply have to control it on the source anyway. In my case it gives this kind of volume boost if compared with my old motherboard; 43~44% ampless provided roughly the same with ZO2.3 @ high gain and source set to 19~20% so it will give a decent boost in vol, the ZO2.1 that I use is even slightly louder, has one only one gain setting but volume is controllable though only needs source vol set at 14% to sound equally loud so yea it's even slightly more powerful but the upcoming ZO3 is said to have even stronger amping.
Well the lower you can run the source volume the cleaner the output will be with ZO and ofc it depends how high the bass boost is set, the higher you go on the bass boost level the lower the volume level might have to be to sound 100% clean (this will vary depending on headphone and source used), with M100 there shouldn't be any issues but I can't speak for dangerously high volume levels as that's not my cup of tea.
Just one more thing, make sure you don't use any EQ settings to boost bass with ZO, better let ZO do the work alone and you get better results. Minimal EQing is still okay, like cutting some midbass if you find the midbass too bloated for example or the mids/highs if you're unsatisfied with how the M100 sounds like there but yea try to let ZO do as much work as possible.