It's all about design expectations. Given they have a high impedance your Sennys are presumably designed to be used with higher tier sound cards or a separate headphone amp. The X-Fi is more of a mid-tier card, so will be designed to work with more common lower-impedance headsets (32 ohms looks to be pretty typical). Put the two together, and you're not going to get optimal performance.
Of course, if the senny is comfortable and you don't want to splash out on a new headset you could buy a headphone amp to sit between the soundcard and the headphones...