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The difference between the denis wick 4ay mouthpiece for the Yamaha euphonium and bsm4x is a different rim shape which makes the bsm4x feel more comfortable. Also the cup is just a little less shallow than the 4ay. Is that for the high register? They both fit the Yamaha lead pipe.

After playing on the denis wick bsm4x mouthpiece on my Yamaha 321 I feel my high regester improved as well as my stamina. I feel that the bsm4x is an improvement over the 4ay.