If I remember my Marantz line correctly, Varimatrix at it's extreme left setting provided L+R mono to the rear speakers, stereo at it's center position, and L-R at it's far right position.
At the far right knob position (complete L-R), if the source material is good the rear speakers are clean and artifact free. Great ambience extraction. The 2 or 3 O'clock knob position adds stereo presence to the rears, at the cost of increased in-phase crosstalk.