Hi
@stormers,
Can't comment on the Xelento, have not tried it nor not interested in Beyerdynamic in the least so this will certainly be biased.
However, I have tried both M 7 (I own this) & SE 846.
This is highly personal & subjective but after extensive demos over the years, especially with the SE 846, I eventually picked the M 7 which personally has better overall sound across the spectrum & frequency range which is more
'true' to the music or sound source as the artist intended.
After all, M 7 is classed as monitoring IEMs, though I'm sure some will disagree.
Keep in mimd, source synergy matters with this IEM, as it does with the whole Sony IER M Series IEM range.
So any DAPs &/or sources which are neutral &/or flat will not play well with the M 7 in particular though this is very true with others in the IEM Series, especially with the IER M 9.
Astell & Kern, Sony DAPs (almost ironic) are among those DAPs which the M 7 will accentuate the DAPs neutral &/or flat tones, no matter what music you play so everything may sound 'lifeless' or at least unexciting...
Your mileage will vary, of course so a demo is highly recommended & cannot be overstated enough, especially with different sources.
Just curious, what are you upgrading from ?
Hope this all makes sense, feel free to ask more if you need.
Hope you have a great day !