I have the same problem. I have a large 15x22x12 room, and somewhat reflective, especially high up. I am getting old and my hearing and dialog comprehension is going away with age. However, I have carpet AND and front rug, and my back wall doesn't exist (open to another room). I use Definitive Technology BP2000s for mains and CLR2000 for center from late 1990s (older DT stuff is quite a bit better than anything else they made after that). And I use a Yamaha RX-3070 receiver and YPAO. But I have trouble with dialog, especially in high dynamic range movies with dialog mastered at a lower than usual level where I have to turn it up and then the loud parts are just way too loud. Here are my suggestions for what worked for me, in order of priority. Start at the first and work your way down, you don't want to waste time on the last part of the list if the things on the first part of the list aren't tried / addressed first.
1) Make sure your center is aimed directly at ear-level of your MLP. It might need to be angled slightly up or down. Sit there for all the steps below. And first get MLP to work well for you by trying all the steps below. Only then, once you have good results at MLP, then move on to tuning other seats, if still necessary. And make sure it is really pushed all the way to the edge of the table, maybe even sticking out a hair past the edge.
2) Try simply adjusting dialog level in the pop-up menu. Option->Dialog->Dialog Level and DTS Dialog Control for DTS sources. Also make sure Dialog Lift shows off (--), as it can spread the dialog across all three front speakers. You don't need that in your setup and it can muddy the dialog.
3) Check your Yamaha DSP soundfield setting. Turn off soundfield (use straight, optionally with Dolby Surround Upmixing on: press the surround decode button once after pressing straight, it is a cleaner DSP/upmixer for dialog). If that doesn't help, try Pure Direct mode and see if that helps. If it does, something with your YPAO settings is causing the problem.
4) If you have the ability to compare: compare a source like a Blu-ray with Dolby True HD (+Atmos if it has it) or DTS HD Master Audio (+DTS:X if it has it), which is uncompressed, to the streaming version of the same like from Netflix, that will use 2.0 or 5.1 (or Dolby Atmos if you have the Ultra HD plan) over Dolby Digital Plus (DD+), which will be highly compressed for streaming. If the streaming is worse, you are sensitive to compression artifacts like I am. You'll need to make adjustments for compressed audio. I use two speaker configurations in the Yamaha, one for movies with uncompressed audio, one for streaming movies with compressed audio. If that's the case I can share the details with you on how to change your setup to compensate.
5) Some stations / sources / devices (like Fire TV Stick 4k and local channels on my Dish Hopper 3) erroneously send 2 channel PCM as DD 5.1 with only two channels active (L+R). This will bypass you AVR's ability to detect and decode Dolby Pro Logic, and will spread the center channel out into your left and right speakers. This is the worst case for me, dialog becomes very unintelligible except at the perfect sweet spot of MLP. Force your source to 2.0 PCM instead for that material and that will properly engage DSU which will do Dolby Pro Logic IIz like extraction which will center most dialog to the center channel speaker.
6) Get a rug on that hardwood floor in front of the TV. If you don't have an appropriate sized rug, just throw down a blanket. Adjust the size of the blanket to see how various sizes rugs would affect dialog ... spread out to cover in front of the center, and then all the way out in front of your mains. Try different blankets, some are more absorbing than others. See if that helps with early reflections from the center speaker from floor. Also, hang something (with low adhesion painter's tape) to the wall under the TV behind the center speaker and also on the wall behind your mains, and also place the center on a towel (I know you have it pushed forward, but the surface under it can still add reflections). If any of that helps, some acoustic treatment / rugs will be helpful.
7) For wide dynamic range source material, turn on Yamaha's dynamic range compression. Then you can raise the volume (and overall dialog level) without raising the maximum volume to levels that are too loud.
8) Try with and without YPAO Volume and Adaptive DRC engaged and see if that helps. It will compensate with EQ for various listening levels to match human hearing response. I like both on.
9) If the problem is mostly with male (lower) voices check the integration of your center with your sub(s). With the couch at the back wall and a less-than-capable center a lot of male vocals will have a good portion run through the sub and the back wall reflections will make it worse. Move your sub around the room (Google "subwoofer crawl") and see if that helps. Move your MLP away from the back wall (temporary) and see if that helps. If it does, acoustic treatment on the back wall will help. Dual identical subwoofers might help too.
10) Check the speaker crossover setting (for the center channel if your AVR supports individual crossover per speaker which the higher-end Yamahas do). If it is higher than 80Hz (what YPAO detected) then you have work to do. You may need to upgrade all speakers to ones that YPAO detects are capable of less than 80Hz (it will set crossover(s) at or less than 80Hz if all your speakers are capable). Then play with the crossover settings (for the center or for all speakers) between what YPAO detected but only up to 80Hz. You don't want to go higher than 80Hz as you can then localize the sub and the portion of male vocals above 80Hz that can be localized will be spread between the center and subwoofer.
11) If all else fails, get a quality monaural headphone that's comfortable and wear it in/on your best ear and drive it from the center channel preamp out (yes, you can drive both a speaker with the speaker connector and something else with the preamp output at the same time). Get one with human vocal filtering/focus, zero delay with small adjustability (so wireless is probably out, but you may need a small delay to compensate for the difference between the headphone and center channel distances), and that doesn't block outside sound like on open design. Then you can adjust your personal center level as you please and hear dialog clearly and still enjoy immersive audio. Consider it your immersive audio hearing aid.
Try combinations of these things because one can affect the others. I know that's a lot of combinations. You can try sticking to varying things that clearly do make an improvement for you, and skip varying those than don't make any difference.
Good luck and please follow-up and share your experience, especially if something worked so that others can benefit from your experience.
All of these things together worked for me and made a huge difference (except #11, and I'm researching that now).