sorry for the late response. i'm not 100% sure where the quality degradation comes from–it could either be OpenGL's decompression of DXT images or the DirectShow playback.
the best way to test if it's an issue with the OF implementation (Direct Show) would be to save a particularly problematic frame as a still image, then export to a DXT file, then use OpenGL's DXT display functionality to decompress and draw the image.
you'll find the code to display DXT images in the HAP addon. reach out if you have any other questions.
oh–another thing to test is whether or not you're using snappy compression in addition to DXT (see your export settings in Adobe Media Encoder). using snappy could lead to more artifacts.
you can also try using the high quality HAP codec and see if there are any improvements.
hope that helps!