What Sonos product I can add to improve dialog

In case you haven’t (and it’s not clear one way or the other), there is a minor feature in the app, as detailed in the speech enhancement FAQ. It may not be as “drastic” an improvement as your looking for, though.

One thing to double check, while you’re looking at this, is exactly what sort of signal your PLAYBAR is receiving. There will be a marked difference between a true Dolby Digital sound, and just a normal stereo signal. In order to maximize the effectiveness of the PLAYBAR, it needs to be receiving a Dolby Digital signal, which then uses that center channel to its best. 

Since the PLAYBAR can’t communicate back across the one way optical signal it receives, it can’t communicate back to the TV set that it exists, and what signal it should be receiving, as the Beam and Arc do across an ARC interface, so you need to check in the Sonos controller what sort of signal it is receiving, and then, if necessary, make adjustments to the TV, and any device that is feeding the TV. 

It has come up frequently on these forums where the PLAYBAR is only receiving a stereo signal from the TV, which causes issues for the listener, particularly in hearing that center channel, since it doesn’t actually exist in a stereo signal :)

Tweak the EQ, Apply the app dialogue enhancement setting (which is basically a narrow range eq boost combined with a low end ‘cut’) are only options options I think. You could also maybe look at placement. Is the play-bar at the optimal height to your listening position? 
 

As already stated - only other Sonos upgrade is the Arc which has dramatically better dialogue (to my ears anyway). 

There is a suggestion topic around here asking for independent volume adjustment for the center channel. After applying all the other tweaks that looks like my last hope.

or a hearing aid? (Not being sarcastic)

Hearing aids are of limited use, even fancy ones with multiple settings are of limited benefit in many sound fields. I only have experience of three users wearing them and two brands personally and they help a great deal. So does going to night-mode and speech enhancement. Tweaking the tone controls also helps a bit.

I have far better luck turning off the Sonos and just using the Bluetooth transmitter that came with the hearing aids when hearing dialog is critical. Needless to say that sucks for the audio experience that my Beam, surrounds and sub provide. Sonos and the hearing aids togethergets into problems with delayed sound, again making dialog harder to understand.

What does help is taking a thick blanket and covering the right and left Beam speakers. Looks kinda ugly and having a simple volume adjustment would be far better.