I just purchased an M-One / M-One XL for use with a guitar preamp, for live use and direct recording. I love the sound of the reverb reflections, but the unit seems to color/brighten the dry guitar sound, even when bypassed.
When set up properly, the M-One / M-One XL will act completely neutral when bypassed and will not color your sound.
If you are passing any dry signal through the effect loop, alongside the signal from the M-One / M-One XL, please make sure that the effect mix in the M-One / M-One XL is set to 100%. Otherwise, you'll get phase correlations between the dry signal, and the dry part of the signal from the M-One / M-One XL .
This is due to the 2 ms processing delay in the M-One / M-One XL.