We get these center speaker problems again and again. The reason, I'm pretty sure is poor speaker design and resulting sub standard performance. One of the best loudspeaker tests is whether not it can reproduce easily intelligible natural speech. Far too many speakers fail this test. In this day and age that is a total disgrace.
All my three systems produce clear natural human speech without shout or sibilance, with NO boost to the center level. No Eq is used either. Heck, I can hear my wife playing our great room system at normal levels on the upstairs landing one floor above. Every word I can hear clearly. Unless you are in a room with the acoustic of a public lavatory, that should be a universal rule and requirement. If you hear people normally in your listeningroom, then you should hear speech from your speakers equally well.
In the time I have had home theater, I am yet to have any family or friends complain that they can not hear or understand the dialog.
People should expect and demand more of speaker manufacturers. A speaker that can not reproduce clear and natural speech is a BAD speaker period.
If you can not demo a speaker and buy on line, and it does not meet this basic requirement, then send it back, with a cryptic note: - "Not fit for purpose."
This is been a frequent complaint for years now, and members here should not tolerate it. With the myriad of design and testing programs, there is just no excuse for it.