I recommend reading this thread for information on PC based measurements. The most likely reason why you get poor SNR is that there is a ground loop between the ODAC and the X-FI HD USB, since both are connected to the same PC. If that is the case, try using a USB isolator, or two separate isolated computers (e.g. a PC and a battery powered laptop) for playback and recording.
A simple example that shows how much difference a grounding problem can make: with a direct loopback between my Xonar D1 and STX (in the same machine), I get a noise floor of about -92 dBA. Adding a differential amplifier to eliminate the ground loop improves that to better than -113 dBA (the Xonar D1 is advertised as having an SNR of 116 dB, which is a realistic value once the additional noise from the differential amplifier and the ADC of the Xonar STX is taken into account).
Edited by stv014 - 9/1/12 at 7:17am