I spliced the front channel RCA either L or R to my sub input, so it gets the signals from say playing music through PC streaming, because it's not in 5.1, thats how I get sound to my subs.
There was a thread about the music player not playing mp3s when you have 5.1 setup, the bass is gone to people that have sub separates.
I run optical from xbox to my receiver, then I took the signal from front channel L and ran it to the sub as well, I did it ghetto style, as I havent gotten couple Y-adaptors/addition RCAs so I can "splice" a front channel signal to the subs.
I am not sure if you guys are talking about the same thing, but XBox (Optical) to my Sony Receiver with 7.1 and sub, I had to do what I said to get Music Player to have subwoofer "sounds" coming through, In game everything is fine, because the games most of them output 5.1
I am strictly talking music cds/streaming music through XBox dashboards music player