It is set to 16-bit (default)
I'd recommend setting it to 24 bit, but that shouldn't be causing your problem.
Three suggestions:
1) If you look at "output format" in DSP studio, what are you sample rate settings like? Are you you up-sampling or down-sampling? Sometimes, I've had strange results when these settings weren't just right. This is unlikely to be your problem, but it's worth a look.
2) Also, in the "output mode settings" dialog box under Audio Options, have you tried using the ASIO diagnostics button? What does it say? Are you sure the correct ASIO driver selected in that window?
3) Do you use the Asus Control Panel at all? I've found that sometimes anomalous problems can be resolved by turning off all processing in the control panel by clicking the "HF" or "Hi Fi" DSP mode. This turns off all of the Asus control panel's proprietary processing. I usually use the low DPC version of the Unified drivers so I don't even have to run the Asus control panel at all.
If none of those is the culprit, I seem to recall having some kind of playback problem after I re-installed the unified drivers on top of the old Asus drivers. I had to completely uninstall all audio drivers and use a third party program (driver sweeper) to remove all the registry entries; then I did a clean install of the unified drivers which resolved my playback problem. See here for more info:
http://brainbit.wordpress.com/xonar-driver-cleaning-guide/