Thought I ought to bump this with the solution I found, because I'm quite happy with it
. And because future googlers like it when people do this.
To keep it short, I'll spare you the part where I tried my Sony HT-CT60 soundbar + sub and a friend's Vizio soundbar in most of the rooms I'm trying to fix. Suffice to say they were much better than the TV speakers, but not really the improvement I was hoping for in terms of dialogue clarity. Basically, they suffered from the same problem as the built-in speakers - turning up the volume turns up the volume on everything, and the dialogue clarity tends to get lost in the noise.
Somehow or other I ended up looking at the ZVOX line and after reading the descriptions and some reviews I decided it was exactly what I was looking for. I got the two units at the bottom of the lineup, the Soundbase 220 and the Soundbase 320. Put the 220 in the kitchen and it's really unbelievable how much better it sounds. I really can't put it into words how much clearer and "more natural" the voices sound, but just to give an example, on multiple different occassions during the first couple of days I was 100% positive that someone was in the house talking. Absolutely sure of it. I had to get all the way to the room with the TV on to realize it was just the TV. With the built-in speakers, there is zero doubt that it's the TV on in the other room and not a person talking. With the ZVOX it's really hard to tell (depending on the content of course).
So, the 220 was a smash hit with my parents - all news programs sound fantastic and crystal clear, and the announcer voices are even almost verging on soothing (I said almost...). The only negative is the size of the unit and the requirement that it be supplied with power and signal via wires (the dreaded WAF, except here it's MAF for mom). The TV's are all wall mounted and the 220 and 320 basically require a piece of furniture to sit on. The ZVOX literature recommends you put it under the TV, with both sitting on a cabinet/desk. So I'm trying to figure out some way to hide the wires with a combination of a little trim strip I'll make plus some kind of box or knick knack to adequately hide the box and wires. As far as I can tell, a wall mount bracket is probably impractical, though I haven't contacted ZVOX about this.
The 320 is also great, just bigger. I'm sure the reason these sound so much better than even the soundbars is their size and the relatively large MDF cabinets. As they say, there's no replacement for displacement right?
Also, there is also a Dialog Emphasis mode - it basically sets up a filter that - you guessed it - emphasizes dialog. It works quite well, but I don't even find it necessary. Just another tidbit to show that these units are MADE with dialog clarity in mind, something that can't be said of soundbars.
In their bedroom, they have a 56" Samsung flat screen that pops out of cabinet and rotates once it's out the top. When it's down, it's completely hidden. Amazingly, this setup has continued to work for 2+ years even though it looks like the type of contraption that would be constantly broken, misaligned, etc. Due to size constraints, the only possible external speaker for this is a soundbar. My Sony soundbar happens to fit the available room in the cabinet like a glove, without impacting any of machinery for raising/lowering/rotating the TV. So that is the new home for my soundbar, with the sub under their bed. This works quite well and is a dramatic improvement in overall sound quality, even if the dialog could still be better. The acoustics in this room are the least poor, so it's a good place for soundbar vs a ZVOX.
Currently the 320 is now mine, and I really like it too for a bedroom system where a full 5.1 system is overkill (for me anyway). However, I think I want more bass than the 320 provides so I'll try out a couple of the higher end ZVOX units knowing that they'll find a good home under the TV's in my parents' other place if I don't like them.
In summary: if the most important requirement is BEING ABLE TO HEAR WHAT THE F*&*^$G ANNOUNCER IS SAYING then one of the Soundbase units is probably perfect. Added bonus is simplicity of setup. Only negative is that the size of the box dwarfs most soundbars. Also, you won't get the bass that an external sub can provide, but you already knew that. They are also a little more expensive than a soundbar, but they are also unique - as far as I can tell there is no competitive product.
I think these are a God-send for the millions of aging TV watchers out there who are slowly losing their hearing at the same time as their TV's get worse and worse speakers. Even the WAF will increase if the W can be convinced into a demo evening, periodically switching back to the built-in speakers which will sound like fingernails on a chalkboard after the Soundbase has been on for an hour.