I've has DISH HD for about 6 months now and all those channels you mentioned are crystal clear for me. I have viewed some of those channels on Comcast from a friend's house and I feel Comcast compresses more. Also, MOST of the Discovery feeds are usually in Dolby Digital, and also HDNet movies are ALWAYS broadcast in Dolby as well. When I flip channels my Denon lets me know what type of signal its receiving from the Dish 811 box. If its in stereo, then I use DTS:Neo and obviously leave it alone if its in Dolby.
Just today was watching Discovery and still enjoying the super clean images for the nature documentaries. Something sounds odd if you're not getting a Dolby signal from movies in HD (which is HDNet, HBO-HD and SHO-HD). Just let the tech come out and see if there are any problems.
