The 1050 should be able to use the sub with the 5.1 direct input. There will be cases, though, when the sub would not get any signal through the 5.1 direct -- because this input bypasses the 1050's bass management, any source that does not include an LFE track will not send the sub anything at all. Are you using the 5.1 direct with a DVD player's internal decoder, or with a DVD-Audio player? In either case, double-check that the source is a 5.1 track (DD2.0 -- a digital copy of a Pro-Logic soundtrack -- may not include anything for the sub to do). You could even try connecting the sub directly to your source to see if you get any signal that way.
Let us know if that doesn't get you on track.