51D and SM3X are two mouthpieces that I have used a lot over the years and with which I have found myself best. A few years ago I come across K&G mouthpieces and since then my search for the ideal ( for me) mouthpiece has ended. Being able to make a comparison the K&G+ has more similarities with the SMX than with the 51D, but with improvements in all respects, especially for the resistance, moreover it more consistent in terms of intonation both in the high and in the low register. K&G mouthpieces have a single edge shape for all models and sizes that is particularly wide, compared to those of the other brands, but also rounded in order to allow great resistance combined with superlative flexibility. It has a 7,4mm throat, quite wide similar to the SMX, while the 51D has a 7mm throat which, in my opinion, makes it more suitable to be paired with uncompensated euphoniums ( the 51D).
For further information on the subject, I invite you to take a look at the article I published on this forum.
http://www.dwerden.com/forum/content...um-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.