TubeNet

Bell flanges are specific to each brand and model. Certain Conns have interchangeable bells and King and Conn sousaphones were the same at one time under UMI (other than the 20K) so they could share bells. A number of sousaphones were made by the same manufacturer at one time under different names, so they can also interchange bells, but in general you can't just pick up a sousaphone bell and expect it to work on any sousaphone.

That said, some horns were made with different bell sizes during their production runs, either as options or just differences in model years. For example, my Conn 40K has a 24" bell, but I have seen these with 26" bells. A 24" bell from a 38K or 40K would fit on a 38K or 40K that had a 26" bell. Some have claimed that a smaller bell "rings" less when played, but I didn't notice a huge difference in that between my 20K (26" bell) and 40K (24" bell). Otherwise, the small difference in bell diameter has little impact on the sound or intonation. The rest of the horn has more impact on those.

This is a very good and interesting question. I would like to see what the other Tube-netters have to say about this.

I have found from personal experience that some older Sousaphones that have smaller bell flares like Martins allow me to better control and don't overblow as easily. But this is just personal experience. Someone else may have different results.

I have perceived no difference of tone between 24 and 26 inch rim, but I do know the 24 inch bell is much more maneuverable. Of course, the bell throat on, say, a 38K and a Conn Cavalier (14K predecessor), and the entire instrument has a different tone.

I really wish the manufacturers would go back to offering a 24 inch rim for the sake of middle and high school players, and especially for those schools whose budgets don't allow for separate tubas and sousaphones.