I have to think that there is something wrong with your hearing or set up. I have a hard time understanding why it would be hard to understand dialog with your speakers.
Turn off your sub and make sure there is no Eq turned on in your receiver, no dynamic Eq or anything like that. Also listen in two channel pure direct with the surround speakers off. See if you can hear the dialog. If you can then start turning up your sub gradually. Stop at the point where is just starts to obscure clarity and turn it down a bit. Once you have that set, then turn on the surrounds. I have to think you sub is turned up too high, which is very common, you have some crazy Eq inadvertently turned on, your surrounds are too loud, or you have a hearing problem and need to get an audiogram.
I don't see any other possibilities.