Great catch, Kevin: You have a brain like a steel-trap!
After contacting Crown about this, Thomas Lureman at the Harman Group said as much.
Here's the brunt of his reply; I don't think he'd mind me quoting him here:
"Unfortunately, I don’t think this will work like you want. The knobs [on the amp] just control the mix, with the same signal level being sent to both channels of the amp. I have this setup at my desk right now. If I turn ch1 knob all the way up and ch2 knob all the way down, both speakers get the same sound information at the same volume level...No matter what you do, in this mode both channels get the same amount of volume.
I think you need to mix the signals together before the amplifier. Your mixer should be able to do this. Then, put the amp in stereo mode and link the input channels together. You will have the same sound information on both channels, but have level controls for each room. This mode is ideally made for bridge mode, where there is only 1 output and a stereo input."
So, per his most gracious and insightful input, I went back to the think tank and the mixer, then it hit me:
1. Run the left and right channels of your stereo input respectively into "ONLY THE LEFT CHANNELS" of two different stereo input channels on the mixer.
2. Since both those left-side channels are considered mono, the mixer routes both of them to each mixer main---left and right---the initial stereo mix is now on BOTH mixer mains.
3. Then in the amp DSP menu, set the amp in the "Y" input section to "STEREO" mode.
4. Now you have each mixer main---which have both stereo input channels on each of them---going into the amp, and the amp sending those two separate channels out in stereo, effectively giving you a stereo mix on Ch1 and Ch2 amp outputs.
WALA: That gave me volume control at BOTH the mixer mains and the amp mains!
Thanks, Kevin for getting back to me on this.
I think you would have eventually given me the solution after I wore you out over it!