I am inclined to say that the bell piece and how it is constructed and its material is the most important. Do you agree? But how much is the influence of the slide piece (inner and outer tube) and its material? How much the player and the mouthpiece?
Generally I am looking for a trombone that has character in its sound and has a little bit of "sizzle" if i push it. But the slide has to run smoothly as well. So can I go hunting for a bell piece that I like (also very old instruments) and organize a fitting slide afterwards or should I see the bell and slide more as one piece that has to fit together and follows a certain philosophy?
Would be very happy to get some inside from you experienced players.
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P.S. The Yamaha YSL-354 holds itself very well against many instruments 3-4 times its price...