No I haven’t detected any bass reduction, and I have previously checked this out in the past. What model of WDTV do you have? I have an older Live Plus and the HDMI output from it goes into my TV’s HDMI and analog out of TV goes on to the stereo amp… I also have the analog audio out for WDTV going into my stereo amp in a different input. In both cases sound is same and fine.
To compare a DVD with a digital copy, it important you compare apples with apples. In this case, you need to compare a DVD with an exact image of it, and that is an ISO file you created from the DVD that you play on the WDTV. If you created an mp4, mkv, etc. your ripping program may have tinkered with the audio track to reduce the bass, etc. to compress the file.
Also, the WDTV is not exactly a high end audio device, but it is still pretty good. It could be reducing the bass a little, too, but likely at the very lowest frequencies where you “feel” the bass, not hear it, per se.