Have you tried using Direct mode? (I think it may be labeled "STRAIGHT" on your remote.)
Direct means the audio input is passed straight thru to the output with no enhancements or alterations within your receiver. Sometimes, many times, your receiver will add or enhance sounds that can actually make voices less clear.
So before I tried doing any additional enhancements, (like equalizer tweaks), I would try removing all stuff that has already been artificially added. Try your Direct/STRAIGHT mode... it is the push of one button on your remote.
(I trust you know when the problem is some millennial talking too fast, or somebody like Kiefer Sutherland who whispers, or some foreigner, or any of the others who talk like they have a sock in their mouth. In other words, even with the best system, best speakers and best setup, some people are still hard to understand. That's why our Skip-Back-10-Seconds and Closed Caption On/Off buttons get used occasionally.