Control Room downmix preset

If I did get that right playback of Preview (Pool, Media Bay and Offline PlugIn) in a setup with 5.1 Main Out is standard to left right channels.
In dialog work when I play a line through a Offline Restoration PlugIn I want it to be played back to my center channel, as that is where it will be played in the mix. The possibility to have different monitor setups in Control Room seems to be a way to achieve that.
I set up monitor A for my mix and B for preview. The Preview monitor gets a Downmix Preset “5.1 to mono”. Now for preview I switch to B and have playback from Offline PlugIn to my Center channel.
BUT…whenever I open any Nuendo session Nuendo remembers what Monitor has been used last, but Downmix Preset is lost, will say back to standard 5.1 to 5.1.

Another way to do it would be to set in a Mix Convert PlugIn in one of the insert slots of Preview monitor. Set that to “5.1 to mono”… But now, through Mix Convert playout will be to my LEFT channel…What do I do wrong?

Thanks for any ideas…