The ground loops worsen the signal-to-noise ratio for such measurements but the harmonic distortions improve. So, the line input of the Auzen X-Fi Prelude is found guilty, and the nonlinear distortion of its output should be made out among the noise.
The third harmonic is very low, and there are no high-level harmonics at all. The Creative X-Fi Elite Pro is formally superior to the Auzen X-Fi Prelude in terms of total harmonic distortions, but the spectrum shows clearly the fifth and seventh harmonics which have a negative impact on sound quality according to some researchers.
There is no sense in comparing the amount of intermodulation distortions due to the great difference in the noise levels. I can only note that the apprehensions about the high level of crosstalk between the channels come true: the Creative X-Fi Elite Pro is better by 15dB in this parameter at frequencies above 1kHz.
Using the ASIO interface it is possible to send the sound to any of the card’s analog outputs. By the way, there is a strange tone at 12kHz of a mysterious origin when the ASIO is in use.
The measurements do not show a considerable difference between the Prelude’s outputs, but the front output is better in every parameter except for crosstalk. These are the results you could expect from an audio card with such topology and components. The rear outputs of the Creative X-Fi Elite Pro are, on the contrary, much worse than the front one in terms of distortions.
I want to note a strange fact: after a while, perhaps after I had used the card with headphones without an external amplifier, the level of low-frequency noise grew in the right channel.
That was because of the LM4562 amplifier. I replaced it and got rid of that noise. The noise is not audible, the sound remains perfect.