Dialogue is in the midrange, not treble- a tweeter won't help.
If you can't hear dialogue, turn the balance to one side and if that helps, do what TLS said and if you can't, make sure the speakers don't have a problem. If you measure the response and the midrange has no particular defect in output level or range, check your hearing. Seriously. As we age, our hearing declines and if you have been exposed to high SPL midrange sounds, you shouldn't expect your hearing to be as good as it was, especially if you hunt, shoot for target practice/competition, work in a loud environment or even if you like to drive with the car windows open. Sometimes, as it was in my case, one ear seemed to have become insensitive to most sound above a few hundred Hertz and it turned out to be obstructed, so I cleaned it.
Also, if you recline your seat and lay back, your ears aren't in a position to perceive midrange properly. Our ears are positioned as they are for a reason- so we can hear voices and sounds that could signal danger. Next time, sit normally or tilt your head so head's axis is oriented vertically- the sound WILL change.
If you listen to music on a portable music source (iPod, smart phone, etc) with ear buds, I can almost guarantee that if you like the music loud, your ears have been damaged by this. I know someone else whose ability to hear dialog is terrible and she runs middle-long distances and always with her iPhone and ear buds. Her hearing is terrible and for some reason, cranking the music while using her ear buds never occurred to her as a possible cause.
FWIW- Koss headphones have come with a warning that sound above 95dB for long periods is harmful, since the 1970s, but I seriously doubt many people have read it. If they did, they paid no attention.