A few additional thoughts,
I was goat roped....errr, "volunteered" to help with sound issues with people of a "certain age". The first was a 71 year-old lady that suffered from some hearing loss from the flu at an early age then age related hearing loss. She wore hearing aids but wanted to watch Netflix without them. Didn't need high SPL or anything, but much higher than her TV could provide and she likes classical music, musicals etc.
Realized that the voice processing or "vocal lift" as some companies call it would work well for her so spec'd a Yamaha sound projector soundbar with "vocal lift" and various EQ settings if she needed more clarity. Just because, I pointed out the Yammie allowed a subwoofer to be connected if she desired more bass. She ordered it, I set it up, she learned about adjusting just the spoken word, EQ and so on--she was thrilled! That honeymoon didn't make it a day so I went back and she wanted more bass. Since it was a soundbar, high SPL was not required but she would like that it played all the "88 Keys" accurately--told her to get the Yamaha 10" sub since it does go down to 27Hz.
It came in, I did the setup and she threw in Bob Marley, some classical and bass heavy dance music. Her 18 year old niece stops by and she remarked her great Aunt had more slamming bass than her dad did.
I also helped a friend in his late 60's to set up his HT system--he wanted to watch concert DVDs and such. His AVR sits at around 100 WPC but his mains are fully horned loaded and hit 103dB at one watt! Figured it best to get him a sub that could have a chance against the mains so he built a sub called "VBSS" which is based on an 18 inch PA woofer tuned to 20Hz with heavy DSP to protect it down low while hammering out serious output from 400 watts.
He really liked the sub... guess what? He wants more! The solution is a second VBSS and it was bizarre being in his house and watching the limiters light up on a PA amp driving an 18...
Grandma likes Daft Punk and Grandpa turned into a bass head with his first subwoofer. The interesting thing about both of them is dialogue clarity, if that did not work then it was an instant fail. The amount of EQ for the guy to be happy was rather extreme--but I suspected that so his center has a horn on it to handle any wild EQ boosting. It sounds very treble heavy with a rising response and will clear the ear wax out of your skull...but it is just him and that is what he wanted.
With HT for folks that required the utmost speech intelligability--the center is the center of the universe. As one guy here on AVS put it "The center is the main speaker with the others just being fluffers". Very true, the L/R just widen or shift the spoken word with the center doing 90% of all the work.
The other factor, you state he is 16 feet away from the center. At that distance, most music or consumer speakers will be such wide dispersion you'll be hearing all sorts of reflections which blur the spoken word. What type of floor covering if any? If he is running bare floors, you will have a ton of floor reflections and if he has plenty of glass windows etc--you might be approaching an echo chamber.
Just because of the distance, I'd look at waveguide or horn speakers--ribbons/AMT/planars can do the same thing. The point is to keep the sound off the floor so a narrow vertical dispersion is key there. At a distance like that, you could use actual cinema main speakers like the JBL 4722N used in small to medium theaters. Way over the top but the massive horn on that thing crosses at 680Hz so a "mid/tweeter" in actual operation and the massive horn controls the dispersion.
The last factor is the worst one--it is your dad, you can't get rid of him, he will be very honest with you and you have one shot at this! Personally, I'd use waveguide/horn speakers since they have extraordinary amount of durability, power handling and efficiency that heavy EQ on the high end won't cause them to compress, distort or blow up. If he puts a +6dB boost say at 4 to 8 KHz, that is 4 times the amount of power which tweeters might give up the magic smoke. For that reason, when dealing with people that require more EQ for the mids/highs than typical--I go for a very heavy duty driver that can withstand heavy boost without complaint.
My inlaws are 70 with hearing problems, they love my HT and always remark how clear the voices are even with the hearing aids out. Yeah, I do a rising response EQ with vocal processing to raise that up also. It is basically at the limit that I can stand but there is no distortion. The center uses a massive waveguide crossing at 950Hz with a monster compression driver that can take over 100 watts by itself without flinching. My mother-in-law remarks how it is the best sound clarity she has ever heard--makes sense, how many theaters or speaker systems are set up for people with hearing problems?
The very large waveguide/horn, a giant compression driver taking care of the upper mids and treble, little floor/ceiling bounce because of limited vertical coverage, carpeting on the floor with vocal processing/EQ worked perfectly for them. When they leave, I reset the processors/EQ and return it back to more accurate settings of course.
If it was me, I would get just a center channel by itself. Run it in mono with the spoken word, say TV news or something. Throw on his favorite programs and so on... in mono. One channel, the most important one that will deliver the thing he demands the most--worth isolating just it. Be very critical when listening, listen or measure for echo, reflections and the like--make absolutely sure that is exactly what he wants speech to sound like--make sure it has the SPL he wants. Then, get the left/right and surrounds to match or close enough to match.
Back in my PA days, I would bring a recording of men and women speaking--just talking. I'd test the various PA speakers for speech clarity first. I'd check the dispersion on an off axis to determine how well they did in the critical midrange. I have "trained/golden" ears with the spoken word, I have listened to it my entire life!
Since people use just microphones for some PA gigs, the spoken word is a true pass/fail test so I use that concept when testing center channels. No music, no boom and sizzle... does the person talking sound accurate and how wide of a sound field will it keep that accuracy?
Explain to your dad first why you are just ordering center channel speakers--he wants it yesterday but doing things the right way take time (my dad told me that!) Once he gets the proper center to suit his needs and after you apply EQ to suit his demands--you are done with the hard part. Bass is rather simple, keep throwing more deep tuned subs at it to taste.
Nailing ultra-clear speech from a center channel at 16 feet of distance for "loud HT action movies?"... that is not so easy. Enter the human element and that throws all the math into the blender.
Good luck and I hope your dad still likes you after the project is complete.