Balancing sound to sharpen dialogue?

Can't understand the words?   You're just getting old

I resemble that remark!

Try speaker placement options 1st. Then might try a Schiit Loki. It is a 4 band equalizer. Can help with taming the lower bass and boost mid frequencies. If you have record in/out jacks can be switch in and out of signal path using monitor button.

@mijostyn: I'm in Spain. If you're in the neighborhood and you've got a calibrated mic, let me know!

@millercarbon: how do you feel about the Marantz NR1200?

I ran into something similar to this a couple years ago.  A friend said he had trouble hearing the dialogue of his movies.  When he turned up the volume, everything got louder.  The voices were still lower.
Went to his home and took a listened.  CD, AUX/ipod In and FM worked okay.  After I got the speaker connections in phase, better.  Popped in a DVD and yes, dialogue was softer muffled mush.  He had set the DVD Audio to 7.1.  "I want the best...."  ?Best?  I set the audio to 2 channel stereo.  Wallah.  I drew him a picture of what 3,5 and 7.1 meant.  Most of the dialogue goes to the center channel.   "You don't have one of them".  

I don't know about your bottom end problem.  I wasn't focusing on that.  But I could imagine turning up the volume make it atrocious.

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.