I use mine with a severne with no problem it you thread it like this...
It will go all the way down no problems, you also upgrade the extension to the 'race' version, that uses all three rollers, and won't twist the pulley, it will even fit on the skinny XT, but you have to thread it, (no loop go) I ordered the part but it never arrived, so I stick with this method and it works find.
This is another method also where you loop the rope from around the other side, it similar to how it would work if you 'upgraded' the extension but the rope does need to go underneath.