That must be a Marantz issue, as the information I found from DIRAC stated that you should be able to adjust volume levels as normal.
As a matter of interest what is your center speaker? This issue is very much a center speaker problem rather than a problem with auto levelling.
Having to increase speaker level to easily hear dialog should not be required, but it seems it often is. However, I do believe this is a speaker issue and not a volume levelling issue per se.
I am lucky as I don't have this issue. I have two systems with center channel and I have not had to increase the levels on either, and I have never had to increase the center volume. We have a couple who we are close friends with and the wife of the couple has a significant hearing deficit, and yet she claims to hear this AV room system fine for movies and other program.
The only way that I can see how you can solve this question, is to get a power amp for the center channel and put a passive volume control between the pre-out and the amp.
Then you would run Dirac with the passive volume control turned down a bit and then turn that volume up, when you need to for issues of dialog.
I just realized that the AV 10 is an AVP, so you have to have external amps. So, you solution is easy, a volume control between the center pre-out and the center channel amp. That will solve you problem for certain.
This is just what you need.