Balancing sound to sharpen dialogue?

jmk2, I live across the pond. Calibrated mics are very common. People who set up systems and theaters use them all the time. Some audio stores might have them. They are not all that expensive. You can get a USB mic for $75 US with a basic program for a PC and get an accurate picture.
It may be your room causing problems or combination of speaker and room. But, the higher frequencies delineate speech (2 to 8 kHz) and the lower midrange frequencies muddy it (200-300 Hz). Make sure your tweeters are working. Trial and error is not the greatest way to go about things. It can be very frustrating. Another option is to get an inexpensive equalizer. 12 bands will do. 

Dynamic range has nothing to do with it. Judging by what you are saying you have too much in the 200 to 300 Hz area and not enough between 3000 and 8000 Hz. The only accurate way to see what is going on is to measure the system with a calibrated microphone and matching computer program. But, you have to have some way to EQ the system,
like a DEQX unit. Bass is accentuated with speakers in corners and against the wall. Pulling them away from corners and the walls may help. Also check to make sure your tweeters are working. They blow easily. 
I have never watched a movie or TV show and had problems understanding the speech. Perhaps someone in your area has a calibrated mic? Where do you live?