Short answer: yes, you can use mains without a sub for two-channel.

Long answer: there are a few ways to do this. The easiest is to leave your mains set to "large" all the time and leave the sub on. This would send full-range data to the mains all the time; the sub would still get the LFE channel and (I believe) the low frequency from the small speakers (center, surrounds) during movies. If you want the mains "small" during movie watching so the sub is responsible for everything below the crossover points, you would want to switch the mains from "large" to "small" between music and movies -- I haven't tried it, but I suspect a couple of macros could be put together to automate this process. (A third option if you are using stereo bypass mode for CD's would be to leave the mains small all the time and turn off the sub when listening to CD's.)

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