Hi Tyll,
Excellent technical article and really helps in understanding the problem. It seems several manufacturers releasing new products are having this problem. John Grandberg mentioned in the comments of the ZDAC article that they also had a problem in their initial production run and had an output impedance of 40 ohms from the headphone out. They have corrected it to 10 ohms now. This could also be a potential reason for John rating his ZDAC(10 ohms) better than the XDA-2, while Steve Guttenberg felt the ZDAC (40 ohms) to be inferior through headphones.
Can I also suggest more technical articles as I feel there is a lot of false unscientific information floating around. Thanks