I would use ReaSurround instead, which has mute and solo, and set up the routing like so:
You'll have to adjust the gain on the LFE and center channel to compensate for their duplication into stereo.
This way you can adjust the 7.1 field and hear the results of that in the stereo mixdown, which means better chance of paletteable phase.
Don't expect to keep all the channels btw, it's not uncommon to totally dump either whole channels or certain frequency spectrums of some channels.