In your opinion, with regard to the high sixth partial, is not a common problem for brasses in general? On all the instruments that I have played all of them, some more or less, they had the sixth harmonic high, especially Eb. Even with my Bach 42 GO trombone, if I play the harmonics without moving the coulisse, the sixth partial is high. Even looking at the intonation chart by Mr. Werden of all euphoniums, except for the Adams, the sixth partial is, some more or less, high.
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.