Understanding words isn't all about loudness. A lot of it is dependent on sibilance - the high frequencies necessary to hear and distinguish consonants such as "S" and "Sh," etc. It's also important in telling the difference between a "P" and a "T." Most stock TV speakers do a TERRIBLE job of this part of the audio spectrum. Making bad audio louder will not fix your problem - especially since high frequencies tend to get smothered by background noises. My advice would be to install a good sound bar (with or without a sub-woofer). You will enjoy everything much more and understand speech better.
Rob