I have a need to do something similar to the result you are looking for; Boost dialogue in movies.
But in my case, I want to do it on a HTPC 5.1 system, and most often with stereo sources, so I am Up Mixing. The problem is similar to a stereo system though.
You will have noticed in the Audio Path that the only changes made to the audio before the conversion (down mixing) of a 5.1 channel source to a 2 channel output are volume adjustments. All Parametric Equaliser, Room Correction, Clip Protection etc. are applied after the Down Mixing. Which means you can't add the Centre channel to Left and Right, because the Centre channel no longer exists. Adding it adds nothing. In fact the Centre, Subwoofer, and Surround channels have already been added to Left and Right via Down Mixing.
If you just want better clarity in dialogue, you can try selecting "Night Mode" under "Adaptive Volume" in the DSP Studio. That does improve dialogue, but it flattens all audio, so I don't particularly like using the solution all the time. Try Night Mode though, and see if it does what you want.
The best solution, I think, is to boost the frequencies of dialogue in the movie, using the Parametric Equaliser. On my Workstation stereo system, playing a 5.1 version of "The Matrix", boosting 100 and 1000 Hz by +20dB makes a considerable difference. But that is a crude adjustment. Night Mode has a larger effect. One day I will get around to asking the experts around here what the best adjustments to make would be. It may also be necessary to use Zones and Zoneswitch to apply any DSP adjustments selectively to different sources.
Anyway, play around and see what you can achieve.