I doubt you would find much difference in the dialog.  It is true though that after you add the sub, the playbase will stop player those lower notes and dedicate resources to the higher frequencies, but I’m not aware that dialog would be played at a higher volume as a result.

I do notice from your settings that you have specified that your surround speakers are 10 ft/3 meters behind your main seating area.  That’s going to result in your surrounds playing rather loud to accommodate for the distance. Might be worth adjusting the TV level down to. I am guessing at this, but perhaps you’re adjusting the volume based on what you’re hearing from surrounds instead of the playbase.  You’re hearing dialog better during action scenes if some of the dialog is coming through surrounds at that time. 

Thanks for clarifying the sub’s issue, I guess it would not be the key to increase dialogue level here.

As for the distance of surround speakers, it has been set by Trueplay and it is now greyed out I couldn’t adjust it if keeping Trueplay on...Good point on surrounds level though, but I didn’t feel much improvement on dialogue even I turned TV level down to -10 and it starts to lose all surround sound. During action scenes, I reckon dialogue is still coming from Playbase, there’re just some loud sound effects (gun shots, swords etc) on surrounds, it might because of different mixing techniques used in action scenes?    

Odd that you can’t adjust the speaker distant.  I see nothing that says it can only be done during trueplay, and seem to recall making changes.  I can’t look at this myself right now as I’m not at home.

https://support.sonos.com/s/article/4804?language=en_US

Adding SUB would improve the clarity of the midrange because PLAYBASE has a simpler task, but adding  SUB will not result in higher dialog levels. Sensitivity to this “improvement” varies wildly from individual to individual. 

Try using the system without Trueplay and also repeat the Trueplay being careful to accurately locate your listening position.

a lot of people have been asking for center channel adjustment (you can find forum topics on it).

sonos has simply said it is under consideration but no time line given  so it may never come. 

I tried to disable Trueplay and adjust the distance of surrounds, and then turned back on Trueplay, now it is 2-10 feet. Guess it works this way?

Yeah, I tried Trueplay with different methods: walk around the whole living room; walk just the TV area; walk just the seating position, but it made no difference to me… 

Another mystery to me is, if me keep turning on Trueplay, would it gradually improve the overall experience? If so, I’d keep it on for while

Yep I have seen lots of them as well. Hopefully they will add this adjustment, I think it’s kind of like an essential feature for home theater 

The challenge is that every sound designer has a differing level for dialog in every single show. If you’re sensitive to that, you’d be making adjustments constantly. 

This is true... the funny thing is, when I rewatched The Lord of the Rings on the Sonos system, I found that the sound of Orc’s breathing and growling is much louder than character’s conversation (barely can hear what they are talking about without subtitles) … So bad guys are always supposed to make much louder noise than good guys (human and elves)? 

How certain are you that you’re getting a Dolby Digital signal to the PLAYBASE?

If I were to guess, I would expect that either the PLAYBASE was getting a stereo signal and interpolating the center and surrounds, or there is an issue with the center speaker in it. Have you contacted Sonos directly?

I’m quite sure because when I play digital TV news, it showed in the app’s control page as below:

Whereas if I play movies from Netflix, it is like this:

Interestingly, the dialogue level from PCM stereo is actually much better for me compared with DD 5.1 content, I watched trailers of No Time To Die and Matrix 4 from YouTube, which is stereo, and the difference between dialogue and action sound effects is acceptable to me.

I did contact support team before and submitted the diagnostics, they said there’s no issue from their end, I guess this means there’s no hardware problems? They also suggested me to use stereo if I think the dialogue level is too low in DD 5.1…. 

In the meanwhile, short of changing over to a solution with a dedicated centre channel speaker with its own volume control, sub titles is the solution. Since I see many not English movies with English sub titles, this isn't a challenge since I am now used to seeing movies that way.

Keep it on if you like what it does when it is on; it has no learning ability over time, assuming that is what you are referring to.

If there is anything that will make this seem to be the case, it is that your brain will get used to its adaptation, and make it seem better than it does now. 

Yes, I am referring to the learning ability over time based on particular environment.. so it is not the case, I’ll try manually tweak EQ settings later

Trueplay does not have this ability. It is a one time thing, the tuning it does. Which is also why it has to be redone if speaker placement changes.