Thanks for all the info everyone, and for taking the time to try to help me out! Really appreciate this.
I reran a Dirac calibration this morning, being more careful about everything, but in the end same result: everything is better with Dirac, the surrounds, the bass, the soundstage, except dialog levels. I tried to bump up the center channel which obviously fixes that, but then I'm having to bump it up by so much (at least 7db) that any pan across the front speakers now feels unnatural (since the center channel is too hot compared to left and right).
I'll have to try to play with it some more. Maybe I need to go into Dirac before exporting the filters and tweak the graphs but I'm not sure what to aim for there.
As for connecting the center channel to the Denon pre out, that's a good idea I'll try that too. I checked and Denon for some reason doesn't offer an option for the center to be set as large (the other speakers can), but I'll keep it at 80hz (small) and connect the pre out and see if it helps.
Once again, thanks for all the help. I know my system and setup is far from ideal but thanks to all the tips I'm making great strides!