I don’t know of any mixer that let’s you play sound through a channel if the CHANNEL fader isn’t up.
Now for CROSS-faders it’s different. Some mixers allow you to assign the A or B or Thru option per channel, allowing individual setting of the Cross fader assignment as you like. In the case of thru it means the Crossfader is effectively not used on that channel’s signal. But you would still need to have the channel fader up in order to get any sound.
By the look of it the M6 does allow you to select one input channel on each side (AB) for the cross-fader, this would theoretically mean that the other non-selected channels will be thru by default.
Hope that clarifies and helps some.