Control Room Downmix Presets for Headphones

Does anyone knnow how to change/edit the downmix preset for the headphones (the “Phones” bus, as labeled in the Control Room)?

I am doing a mix in Quadro (Surround) and it seems that the Phones’ downmix to Stereo setting defaults at -6db for the Surround speakers (Ls and Rs). I would like to change that to either -3 or 0 but I couldn;t find any way to doing that. Altering the “Downmix Presets” in the Control Room Mixer window does not have any effect on the Phones (but it does for the monitors).