I am not hard of hearing but I can empathize with your experience. I have listened to many home theater systems where intelligibility of the dialogue is mediocre at best.
In your case you have a couple things working against you. It sounds as if you have a Samsung Home Theater System that came packaged as a unit. From my experience, those units usually show a power rating of 165 watts per channel. Here is the problem. . .they cheat.
They use 3ohm speakers, meaning they use speakers with lower grade components and very low resistance to maximize the amount of power the amplifier can be rated at (Americans buy on power. . .watts, horsepower, etc)
As a comparison, a Bose system uses 6 Ohm speakers, and most home theater systems use 8 ohm speakers. There is more to a speaker than power rating. There is frequency response, and for volume, there is efficiency. Efficiency tells you how loud the speaker gets when you give it power. Samsung speakers have an efficiency of 87db.
Why is this important?
It means that you cannot switch your center speaker out to another brand as it won’t help.
My initial reaction was switch out the center channel only, to something like a Klipsch C-10.
This speaker has a wider frequency response for more detail, and has a higher efficiency of 93db. This means that at the same power this speaker is 6 db louder than the Samsung, meaning turning up the audio globally would give your center speaker more volume than the others.
However this won’t work, as the Klipsch Speaker has an 8ohm resistance. It has over 2x the resistance of the Samsung speaker. This means the amplifier will deliver less power to the Klipsch speaker based on that resistance increase, nullifying any gain you may have gotten by the increased efficiency.
Other options typically pursued in our application would be to use a digital signal processor to tune the center channel to a setting that fit the space better. However this is a costly option pursued in higher end systems.
The third option would be to use an assisted listening system. This takes the output of your system and tunes it to be received by an assisted listening headset, or a hearing aid with a built in T-coil and receiver. This is used in public spaces to help the hearing impaired hear announcements, lectures, etc. Again, this option would cost as much or more than the $600 you paid for your setup.
To sum up, if you invest even a nominal amount in a new system, purchase something that works in the 8ohm range. It may show lower power, but it is all a shell game, lowering impedence to show “more” power.
Buying a system in the 8ohm range, gives you options to change the speakers at a later time based on your needs/preferences without replacing the DVD/amplifier head end unit as well. In your case, if the Samsung unit was an 8ohm piece, you may have been able to solve this problem for $199 switching out the center channel, as opposed to being faced with buying a whole new system to fix your issues.
Feel free to contact me directly at markc at orangeproav dot com if you want some further advice. . .gratis of course.