Surround to Phones monitoring discrepancies

Nuendo 8. Trying to monitor a surround project on headphones using a binaural plugin, I was quite puzzled by a level difference between pan positions using the VST multipanner.
So I created a simple setup for the test: one stereo track with the tone generator plugin (pink noise and sine tones) > quad master out > 4 loudspeakers / and headphones monitoring.
When the signal is panned from front to back/surround, the level is equal on the quad master meters (and equal on the loudspeakers). Here all is normal. This is confirmed by the loudness meters.
Now when I monitor on headphones, there is about a 6db level drop when the signal is panned from front to the back. The Phones meter confirms that.
I guess it would be normal to find a MixConvert plugin inserted in the Phones output (in Control Room) to reduce the signal to stereo but I could not find any access to it. (the MixConvert plugins for the loudspeaker channels are accessible).
I checked with previous versions of Nuendo and it is the same.
I’ve searched the forum, the manual and the web and could not find any detail on the Phones channel. Is it locked with a fixed setup?
Thanks for your help!