I have had both the 200 and 500 combo, and in my mind, they're about the same volume through the stock speaker, probably because 1x15s often tend to be about the same volume as a 2x10" and cab volume is identical. If anything, the 500 is perhaps even better at lower volume because it has clearer, more immediate sound. To me, the 500 is better than the 200 because it has the opportunity to be much louder with an external cab, though it works greats at all volumes. I think with just the combo, it's about the same volume. I think there's only one truly compelling reason to get the 200, and that's if you like 1x15s. That's it. When I had the 200, I returned it to get a 500. As for the 100, I feel like if you're going to get an amp that's already that big, then it's no longer a true practice size. I use the 40 for practice and it's perfect.
I think tube amps usually sound better when cranked, but SS amps don't have that issue at all.