For years the same problem and I don't have a solution. The AC3 that must be made to make a DVD always sounds about 8 db too low. When I raise the db within Compressor, the AC3 sounds bad, a lot of gain ups and downs, either it's that or it is very sensitive to my mixing modulations. The aif sounds great, so this is something wrong in the AC3 compression. Any solutions when having to compress an AC3? I can't just use the aiff on the DVD, not enough room.
Don't change the mix levels. In the ac3 encode tabs in Compressor, set dialog normalise to -31db and turn film compression off (select none).
Levels should now be close to the aifs. ac3 will flatten things a bit but with those settings, you are effectively turning the Dolby chip off so it doesn't alter levels or mix dynamic which it does badly.