The X-Fi has an impedance-switching FL/FR output. Plugging your headphones directly into that output is equivalent to plugging your headphones into a sort of basic headphone amplifier. When you plug in your Promedias to that output, it acts as a line level output and the Promedias must then provide the headphone amplification (which they do). Conclusion? You don't like the Promedias' amp as much as you do the X-Fi's. Maybe it's just a shitty amplifier, maybe it adds nasty coloration, or maybe it's just something else.
What I did for a while was run a 1/8" stereo mini male to stereo mini female interconnect to the top of my desk so that I could switch between speakers and headphones without crawling behind the desk. If you want a more elegant fix, then buy a headphone amplifier with a built in DAC. Keep your speakers plugged in to analog out on the X-Fi and connect the DAC/amp to the coaxial digital out.
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