I wouldn't want to fool around with dynamic volume for speech intelligibility, it compresses all the sounds together and might make the dialogue less coherent. When I am watching something and want to hear dialogue clearly but don't want to bother anyone nearby, I boost the center speaker way up, and turn the subwoofer channel way down. This really helps. You might just get a 3.0 or 3.1, and make sure you get a decent center speaker, this will help a lot I am sure.
Here is a setup that can get you there:Yamaha RX-v373
3 x Infinity Primus p153
, $180Dayton Audio SUB-1200
Total right at $500.
For dialogue intelligibility, set up one of those p153 speakers as a center with the tweeter as close to ear level as possible. Aim the tweeter at position where your ears usually reside. Try to keep it upright, ie not laying on its side. Learn the features of the receiver, it should have a easily accessible way to get to the center channel level. Just raise the center channel level when you want to boost dialogue, but not much else. You'll want to turn down the sub or turn it off for this too. That ought to get you nice clear dialogue with everything else toned down.
I recommend getting a sub, because it would be better for speech intelligibility to not run the mains as 'large' and have them try to tackle the bass. The sub here is really secondary, but the one I recommended is supposedly very good for the price, and it will give you a fuller sound. I wanted to fit in a really good dedicated center speaker like this one
, but prices have gone up on that model, and I just can not fit it in a 3.0 or 3.1 budget. Nonetheless, a p153 as a center will be worlds better than what you have now, especially if you set it up correctly. I think this setup would best any sound bar.
One more thing, if you can, try to give the center speaker some free space, so you don't want clutter around it or have it tucked in some kind of nook with a bunch of surfaces nearby. This will cause surface reflections which will interfere with the sound, thus lessening its coherence.