With the PT series, it's more the diameter of the throat and the treatment of the backbore. When I tried out the entire series last year, everything in my preferred cup diameter, all the mouthpieces that were blurbed as being more for bass tuba had a smaller throat, @ 8 mm rather than the larger 8.2 - 8.4 throats for mouthpieces blurbed as being for contrabass tubas.
And it showed in the playing. The smaller throat mouthpieces did sound great on bass tuba, but at least for me, they all went flat in the upper register on a contrabass.
What I usually tend to find is that the F/Eb mouthpieces are shallower. This will help keep the pitch from going flat in the high register. Yes, you can play a helleberg on an F tuba, but you may be working harder pushing the pitch up without realizing it.
I personally find that a shallow bowl mouthpiece (rather than a funnel one) will help ME keep the pitch where it belongs so I don't have to work too hard. Jens Bjorn-Larsen used to use a full sized Conn Helleberg on all his horns. He has been using a PT65 on his F but is currently having a custom, signature mouthpiece made. He found that, yes he could do it on the Helleberg, but it was an awful lot of work. Now it is a lot easier on him and he can put more energy into making the music.
Just my observations.
Roger
"The music business is a cruel and shallow trench, a long plastic hallway where thieves and pimps run free and good men die like dogs. There's also a negative side." Hunter S Thompson
