I use the Arturia V3 all the time without problems and also have a stereo/mono switch in my master output. Pianos are panned across the keyboard so if you are taking just a single channel out you will be gradually missing half the sounds. You must correctly mix and adjust the levels to get a true mono output. You will lose all stereo effects.
FYI this is my Global Rackspace with the MONO wiring. Ignore the missing plugins they are not on my windows machine. As expected you will need to make a 3dB reduction in the mono out to keep levels the same. Clicking the Mono LED on by the master output (this is a grouped widget with the other instance hidden), switches stereo/mono with the same levels.
