i use it a 0db since i don't want my M51 to mess with the signal and (to digitally amplify it).
guys on SNA reported some strange issues with m51 clipping on some loud recordings. well, they said that using -1db solves that issue.
considering this info, i guess it's not a good idea to set higher volume than 0
(personally i didn't have any clipping on M51 yet)