Setting up a sub for 2 channels is too easy may be that's why it wasn't mentioned anywhere.
Just connect the extra line out from DAC-10H (either RCA or XLR) to your sub. Then adjust your sub's cut off freq and volume to get the best result.
Oh well, if you want to use the sub for home theatre and 2 channels, that will be tricky. The sub channel from movie is encoded with other surround channels so it is only available in the sub out from home theatre.
I don't think this is possible. You have to get another sub.
By the way, I never like the extra bass from sub anyway, always seems out of sync with music. Better to get a good main speaker.