You mention re-calibrating ARC, so am I to assume that you are running an Anthem receiver or pre-amp?
It would be far better off if you are, to provide a copy of the speaker calibration printout. as it's run in Windows, if you load something like CutePDF if you don't have a pdf print or aren't running windows 10.
From the few times that I have played with ARC, I found that the defaults didn't even come close to letting the speaker run as full frequency. There is quite a bit of configuration that you can do inside of ARC. For dialog you will need to give your center speaker a bump around the 120hz to 220hz. it could be that your room is sucking those frequencies down and your are loosing your dialog as a result of that.
Yes I'm running anthem mrx510 and using the pre-out to a anthem MCA 50 amp. Your advice to filter of at 120hz frequency is too high? wont that make the sound tinny? I'll upload the print out from ARC so you guys could have a look.
