Hi Denys,
I have all Audio-gd equipment currently: C2, DAC19DSP, Digital Interface, and power filter. I have two of their power cables - one for the C2 and one for the DAC19DSP - both plugged into the -70db outlets on the power filter. I run the Digital Interface power from a USB 3 port right now but I'm in the process of buying the DI Power Supply and another power cable for it.
Originally, I compared the Audio-gd power cables with the standard power cords plugged into a Philips power strip (before I got the Audio-gd power filter) and I noticed that my mostly FLAC based music smoothed out with the Audio-gd power cables. For me, the DAC19 benefited more I think than the C2, but both units seemed to produce tighter bass and silkier highs. The Power Filter made this effect even more pronounced.
I can't say what the Audio-gd power cable will do for the DI power supply but I was willing to purchase it none-the-less given what I experienced with the other gear. I figured it would have cost me just about the same amount or more to build a quality 12 gauge power cord - especially with their current 5% discount - so it was worth the cost to me.
I hope it will prove to be a good choice.
--Larry