I am more comfortable with large diameter mouthpieces on the euphonium. I use 4 mouthpieces K&G that alternare depending on the circumstances: K&G3D new version, 26,7 mm; old K&G 3D to which I had to enlarge the diameter because it had been damaged by falling, 27,4 mm; K&G 2,5E 27mm and K&G 1D 27,7mm. I am better with the wider ones, obtaining the upper register without difficulty. ( high F)
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.