At least for the D200 it wasn't a whole new design it was more an update to the previous like how the D100 MKII improved upon the original D100. This DAC is like a culmination of the previous success (D100) paired with new hardware from the DA8. Since the D100 MKII has been discontinued for a while now, I think he may have wanted to fill in the gap left in the mid-fi range by building upon a previous DAC.
Actually all the dac's in the Yulong line have seemed to me like a logical evolution. The D18 I have is basically the DA8 without the added features for DSD, the headphone amp and the usb circuit. I haven't got the means to try it without buying a DA8, but I have a feeling the dac section of the DA8 and the D18 have a lot in common. It would be interesting to compare the sound quality of the two on the dac section alone. Audio gd is a very different animal as their designs win the award for the highest parts count in a box.