I found this true as well. I was challenged during one of my Quintet rehearsals by our leader not to use my flugelhorn but to try to get a flugelhorn sound out of my trumpet. I used the same mouthpiece, but found that relaxing and dropping my lower jaw really mellowed the sound to the point that my leader was satisfied. I then challenged our acoustic piano player to make his baby-grand sound like a Hammond B-3. He let me down.