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I have tried a number of mouthpieces with my 2341 and have found the Doug Elliott R cup with a 130 rim and an R6E shank to work best for me. I have found the Elliott to have the fastest and most accurate response for me. The "E" (European) shank seems to fit this better than the smaller standard American shank.

I was going back and forth between that and a Dillon R1. I don't know much about the R1 other than it is a funnel shaped mouthpiece that appears to be slightly larger than a Conn Helleberg.

I have had good results with a PT50+. Ed

The King 2341 is refreshingly insensitive to mouthpiece selection. That said, since you are a trombone player, and probably more used to bowl-cup mouthpieces than funnel-cup mouthpieces (Conn Helleberg 120S, etc.), you already have a very fine mouthpiece that fits the tuba and many players well, as posted above. The only reason to get another mouthpiece is if there is a particular detail about the King 18 that just feels bad, or you want the completely different tone a funnel cup mouthpiece gives.

Welcome to the trenches!

Try a Blokepiece Symphony with the "P" shank. A really gorgeous rich sound.