have you tried doing the dolby encode at your place? It could be some sort of phase relationship between L, C and R causing the dip in the center channel. Shot in the dark, but it never hurts to try. If you can make the AC3 at your place without any of these problems, then it is obviously something they are doing. If the AC3 has the same problems when you make it, then it could be something in the mix.
On some encoders, there is a trim for the center channel (I know the Neyrink Encoder has one). Maybe they are accidentally trimming the center channel down 8dB more than they should when encoding?
Audio Engineer - Producer - Composer http://www.linkedin.com/pub/derek-jones/8/986/9b9 http://www.myspace.com/daogkilla "We were working on Raiders [of the Lost Ark]. He [Ben Burt] told me that the sound source for opening the lid of the ark in the last reel was within 20'. I couldn't figure it out. It turned out to be lifting the back off the toilet above the water chamber, and slowing it down." -Tomlinson Holman