In the distance like quieter or are your ears just saying the voice is further away ? I found my M60s could deliver some 3d imaging cues (up/down, closer/further away) that I only heard before on a very few, very expensive systems.
If it's "further away like quieter" I don't know the answer
EDIT - one other question -- are you running just "Stereo / Surround off" or do you have DSP off as well ? I found I get slightly nicer results on my 630 with DSP off as well, although that only works with analog inputs or tuner.
