I have the Xonar D1, which is the PCI version of the DX, and according to my tests (some results are available here), it performs fairly well as a DAC, although I only tested the front channels. It is not great for driving many headphones because of the high (100 Ω) output impedance. The line input is decent, but it has no analog gain controls (so a 2Vrms input will always produce a 0 dBFS output on the ADC, but the signal may be amplified or attenuated digitally by the driver), the input impedance is relatively low (less than 4 kΩ), and the distortion is only really low if the signal is not above about -3 dBFS.
I do not know much about the DSX, other than what can be found out from the specs and pictures.
The DGX uses the same headphone amplifier (which is in fact a line driver only, and does not handle low impedance loads very well) as the Xonar U3 that has detailed measurements available at a well known blog, but it could have somewhat lower output impedance (based on pictures of the PCB only, no measurements are available). The DAC is better than on the U3, at least according to the specs and RMAA results I found.