YouTube has a nice video of the Canadian Brass Quintet playing a Purcell piece. While it doesn't really feature the tuba, you can plainly see the black bell in the video. You may be able to hear enough of the tuba to judge for yourself how the sound holds up.
Canadian Brass - Carbon Fiber Tuba Bell
Still wonder why it has not been done on a euphonium bell. Just a matter of time I suppose.
I just noticed it on this video posted this week:
https://youtu.be/bcAF-ZJfbGg