Documented on web site. From the FAQ:
What is the difference between CDA and SDS Class D Audio Power amplifiers?
The difference is the SDS Series amplifiers have a balanced XLR input stage (as well as the single ended RCA inputs). The SDS also has gain control for each channel and also a little better cooling. The sonic difference between the CDA and SDS is very close with the SDS sounding a slightly more open due to the input stage. You would really have to do some side-by-side listening to both to tell the difference. These are some of the finest sounding audio amplifiers available today.