I played on all three, but the measure 3 that, apart from the size of the cup, can compared with the 4. The SMU has a deeper cup, dark tone, I did not particularly like it, while the SMX has a less deep cup that makes the sound brighter, easier to produce the high register, I used it for a while before discovering K&G mouthpieces.
2007 Besson Prestige 2052, 3D+ K&G mouthpiece; JP373 baritone, 4B modified K&G mouthpiece; Bach 42GO trombone, T4C K&G mouthpiece; 1973 Besson New Standard 3 compensated valves, 3D+ K&G modified mouthpiece; Wessex French C tuba, 3D+ K&G modified mouthpiece.