CB, what you're suggesting won't work unless you have some device between the ASUS Xonar D2X's S/PDIF output and the preamp/processor unit performing the function of Digital-to-Analog Conversion (DAC), which reassembles the digital audio signal (a sequence of on-off data representing digital 1s and 0s) into an analog audio signal (where the voltage being carried in the cable actually tracks the audio waveform). There are a few standalone DAC devices in the high-end audio world, but not so many at the consumer level. Of course, you've got Cambridge Audio components, so you're likely not afraid to spend a little money, but some of these devices can be fairly pricey.
The good news is that you've already got a high-quality DAC built into your ASUS D2X sound card -- the D2X uses high-end Burr-Brown digital-to-analog converters, and can achieve a phenomenal signal-to-noise ratio of 118dB. So your best option is to use the D2X's own analog audio outputs to feed the Cambridge Audio preamp directly. Alternately, if your Cambridge Audio preamp can accept any more digital inputs, connect the D2X directly via a S/PDIF cable, whether optical or coaxial. And if not, see if there are any other sources (like a CD changer-player) that could be 'bumped' from a digital input to an analog input.