You have what I know to be a common problem.
The HT environment is a very difficult one for speakers. I think your issue is probably multifactrorial.
From your description, I suspect your room is live, and may well benefit from acoustic treatment.
By far the biggest issue is speakers, especially the center speaker. Frankly, I'm yet the hear any commercial center speaker that I would rate even passable.
I have agonized and spent more time on my center speaker than I care to admit to.
Now your speakers all have their crossovers right in a key part of the speech discrimination band. Especially for a center speaker, 1500 Hz is absolutely the worst place to put a crossover. Also I do not buy that turning an MTM on its side is innocent. I have been investigatig, and I will state categorically that placing an MTM on its side should never be tolerated. The polar response has absolutely everything wrong about it. It maximizes room ceiling reflections and interactions, and gives very poor uneven sound distribution to the listening area. I'm going to stick, my neck out and say categorically if you do that, you have a compromised set up and seriously so.
The next issue is one I have been highlighting of late, is Q. Unfortunately speakers with high Q and even those with a what has been previously Q in an acceptable range are not ideal for HT. It really requires speakers to be truly highly damped to not interfere with speech discrimination. It really covers speech if speakers are of the resonant variety, which ported speakers are by definition.
Now I'm going to really upset people, but I don't think you can make a truly great reference HT system using ported speakers and that includes the subs,
In my reference system I have no reflex loaded ported speakers, and I think that is a crucial design parameter.
The next issue is that I think the mix of many BD discs may well not be optimal. That may well be because a lot of those very expensive speakers systems in the Hollywood production studios, may not actually be very good. I suspect from the pictures I have seen of them, a lot may be very nasty shouty speakers. I have had a film critic who gets to premier events in these studios on a regular basis. He prefers what he hears here by a big margin. I have a suspicion, the dialog may be mixed a bit low because of shouty monitors.
I have this rig set so that dialog is natural and intelligible with a natural balance. This has been a big undertaking. Even so I can have visitors who are a little hard of hearing. One of our close friends has meniere's disease.
Since I use my rig more for music and especially opera, I do not want my center loud even by 1 db. I hate having to go the menu to alter this, so I have a cheater knob on one of my main control panels that allows me to alter the center channel instantly at will, without out going to any menus or set up screens
This issue is not confined to home HT either. My son and daughter in law recently went to Skyfall at the Odeon Leicester Square. This has always been the UK's flagship theater for audio. They paid extra for seats that were said to have the best audio in the theater. They told me they could not hear most of the dialog. They said is was covered by over resonant sound. My son is well familiar with my obsession with non resonant reproduction, and is certain those were not low Q speakers.
This whole issue needs looking at with much more rigor and science. It is an issue that in my book makes the vast majority of home and professional systems poor for a really good movie experience.
I know this is a harsh verdict and will prove controversial, but I really believe it to be the current and unacceptable state of affairs.