I would recommend trying the K&G mouthpieces: I use them on euphonium, baritone, French C tuba and trombone. After years searching for the ideal mouthpiece (for me) and having gone from Bach 6/2 AL, Schilke 51D, 52D, 52E2, SM3, SM3U, SM3X and DC3 and DC2 with K&G ( 3 and 3+) I found the Holy Grail, exceptional in terms of resistance, intonation, projection and sound. Obviously this is just my humble opinion.
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.