Be very careful. I know you have the pin assignments for the case cable provided by Lian Li, as do I. But I cannot find any official information on the FX card pin assignment although it should be standard audio pin assignments. but is it HD or AC97? The Lian Li cable has a connector for both. If you are plugging the HD connector into the FX card and it is HD audio, then why does it not work for the headphones provided the Pin 1 on the FX card is lined up with Pin 1 on the case cable?
In another thread
the poster gives a pinout that does not match the Lian Li cable.
Again, be careful. If the headphones work when plugged into the back of the Supreme FX card it may be best to use it that way for now until you have definitive information.