Paul,
This is an interesting point!
When I listen to Frankie Trumbauer recordings his c-melody tone doesn't sound all that dark to my ears. Rather, it's on the light side. However, with a vintage mouthpiece a c-melody sounds dark, tubby, and muffled. It isn't at all like Trumbauer's sound! So, what do we mean by a traditional c-melody sound -- the vintage sound or Trumbauer's?
It's important to understand that Trumbauer's c-melody sound is different from his sound on alto. This should not come as a surprise. While the design of the c-melody bore is closer to an alto than a tenor, it's NOT the same as an alto. Thus, the air flow in a c-melody has more resistance than an alto and it's sound has a somewhat muted quality. As much as I love the c-melody, it would be a struggle to play the lead alto part on one in a large jazz ensemble. It simply doesn't have an alto's brillance of tone and level of projection.
The Morgan c-melody mouthpiece has helped me to develop a clearer concept of a c-melody sound. Ralph spent quite a bit of time researching the c-melody and created a mouthpiece that has the proper length and amount of chamber volume for the instrument. Interestingly, it's very close in length to an alto mouthpiece; however, it has a considerably larger chamber than a similar tenor mouthpiece. This piece doesn't have a vintage mouthpiece sound. Rather, it has a tone that sounds alive and projects very well. Depending upon your set up, it can have a Trumbauer lightness or it can be on the darker side. I think of this as being more of a "natural" c-melody sound. I really like this sound. It's different from an alto or a tenor. It's something unique.