My mobo only has 1 pci slot that sits between my sli pci-e slots, thus I can't use it as my 580's cover it up. I only have a free pci-e 1x and 4x. And I prefer the 4x because I don't want my soundcard above my videocards.
The xonar is awesome as I have had the DX before, but it requires a floppy-astyle 4 pin power connector to run, and I have already cabled my machine to perfection. I simply want an audio card that doesn't require additional power.
The best spot would be the PCIex1 above the video cards for better airflow, especially if the PSU is bottom mounted. I think the Xonar DX is OK, it's good, it's low profile, it's cheap, it works in any of those two slots and you can route the small power connector but you don't have it anymore.