If the center channel is connected to the amp from the 990 it will sense during auto calibration that the channel is there and will set its size. It does this to allow 5 or 7 channel stereo. When I listen in 2.1 I am in stereo upsample. Either way, when listening to 2 channel stereo, you should not hear the center channel. I have 5 channels connected to my 990 even though I only use 2 channels and the 990 sensed no speaker but won't allow me to set the center and both surrounds to none.
_________________________
Music system
Model 990/7500/Magnepan 1.6 QRs/Technics SL1200 MK2/Aperion S-12 Subwoofer/OWA3/Sony NS75H DVD
APC H15 Power Conditioner
TV System
Large Advent Loudspeakers/ Polk center/Monoprice surrounds/Panasonic Viera 42 inch/Onkyo HT-RC260/Sony BDP S590/Directv
Home Theater System
Onkyo PR-SC886/Outlaw 7125 Klipsch RF-82 L/R,RC-62 center, RB-35 SR/SL, BENQ HT1075, Outlaw LFM1-EX/OPPO BDP-83/Directv
Harmony ONE
Blue Jeans and Monoprice interconnects
APC H15 Power Conditioner