Two things to be aware of when splitting the 990's pre-amp outputs that may not make it the best approach for everyone. First, if you have surround processing applied at the time, you're going to lose data to the center channel and surrounds - you'll need to have a stereo source and be in stereo mode (preferably bypass if you are worried about also preserving the low frequency data that would otherwise be crossed over to the sub). Second, the main output is downstream of the volume control, which will have an effect on your recording levels.