It could still be the setup, even though you only notice it on some DVDs. Different voices are at slightly different frequencies, so a problem may exist with some voices but not others. It could also be affected by the other sounds in the mix. Do you still have the problem when you play things "straight", with no special processing?
Really, without knowing more details, it is impossible to say what your problem is. It could be the mix on the DVDs as you seem to think, or it could be some processing you are applying to it (adding reverb is something that is probably quite common in some of the DSP modes), or it could be that your speakers or acoustics are causing the problem.
In my case, I have never noticed anything like what you are talking about, but I do not add special processing to the sound, and I have great speakers. I also do not have the specific DVDs you mention, so I obviously cannot rule out the possibility that the makers of those DVDs were incompetent fools.