When you say "LFE Only" you are talking about the 990's subwoofer setting right? The sub cable is running to the Descent's left or right input? If that's the case, the one thing that I worry about is the fact that you are completely throwing away everything between 40Hz and 120Hz on the center and surround channels, plus anything above 40Hz in the LFE channel. Can you try the 70Hz setting on the Descent? That would at least decrease the size of the hole in your low frequency.