Years ago...approximately 40...several tube calls were brought to the marketplace with slanted mouthpieces. I always thought the design had merit. Obviously, the slanted mouthpiece tube calls were not a commercial success. But I really like this one....
BTW, for whatever reason, I cannot open my first post....
For finesse calling with a tube call the slanted mouthpiece is a definite advantage for me.
Look at some of the beveled lip rests on prior posts. It seems to give the same angles or lip contact with the edge of the latex. I agree it opens several avenues that flat doesn't provide.
The beveled lipstop is a help, but you are still faced with the 90 degree angle of the reed unless you tilt the tube.
Quaker Boy made one with the angle back in the late 80s, early 90s. I had a couple of them, they sounded pretty good.
I've never heard anyone that gave good effort to trying the angled mouthpiece on a tube call say anything critical. My surprise is that more tube calls with an angled mouthpiece were not made. With today's CNC capabilities, making tube calls with the angled mouthpiece would only require programing the machine to make them.
I'd like to buy one if there are any call makers willing to make one.