TubeNet

To answer both of your questions... No... and no. To me, there is a distinct difference in the sound of American/British and German horns but it has little to do with whether the bell rings or not.

I'm guessing it's the difference between valve types and the bore profile that makes all the difference. In point of fact, people have replaced on bell kind with the other out of necessity and not seen any difference.

With a very tall bell the sound comes out farther from your ears, so you hear a bit less direct sound and more that is reflected off the ceiling and walls. That may influence the way you play, which in turn may lead listeners to conclude that a tall-belled tuba makes a bigger sound. Personally, I prefer a short bell. I also prefer to put my bass amp directly behind me for the same reason. Nobody ever tells me that my sound on either instrument is not big enough.