I'm running into a bit of a different problem lately with DD 5.1. For months we have been struggling with center channel clarity. It isn't the amp, it isn't the calibration, it certainly isn't the speaker ($300 Paradigm). I've become convinced it's the mix.
For most series we have noticed that the dialog is just very poorly mixed against the special effects, and the clarity of the dialog, even with the other channels greatly attenuated is not very good. What I'm observing, is there is a LOT of intentional (for dramatic effect) mumbling going on.
Either that or my hearing (and simultaneously my wife's) is going in the dumpster.
There are two things I have done to reduce the impact of the problem:
Turn off DD 5.1 (in the amp, not the D* box) and run either of these two "modes":
Stereo
All Channel Stereo
Either of these produce much better dialog. I have also noticed that there are few shows where the dialog mix is much, much better than others, in particular NCIS: LA.
Since many of the shows we like are dialog intensive, we have taken to not using DD 5.1 much any more.
(NCIS, White Collar, Royal Pains, Burn Notice, Rizzoli and Isles, Covert Affairs, Suits, Necessary Roughness (which is particularly bad much of the time), Franklin and Bash, Common Law, The Glades, Longmire (can be very bad), Eureka (usually very good), and Pawn Stars (usually very good). Lastly, The Herd is almost always very solid (but it's all talk and no real mix))
It doesn't take a rocket scientist to see that in the above series, special effects take a big back seat to clarity of dialog.
(...and yes, I have played extensively with channel levels with an SPL meter and pink/white noise generator...I still see garbage in - garbage out. No amount of level adjusting seems to make up for poor mixing/miking/dramatic mumbling in the first place, making DD 5.1 and above of marginal value except in "special effects" dominated movies.)