OK, which SM6? There are three similar Denis Wick mouthpieces that go by similar names.
The original SM6 small shank is Steven Mead's first design of a British Brass Band baritone mouthpiece. It has the same shank as a tenor trombone and American "hybrid" bari euph instruments like King, Conn, Reynolds, Olds, Martin, etc. But it still has the larger cup, throat, and backbore that is characteristic of Denis Wick euphonium mouthpieces generally. This may be the SMB6 offered below.
The upgrade is the SM6 Ultra, and comes in all three shank sizes (British brass band baritone, Pre-1975 "medium" B&H/Besson shank, and modern large shank. I have one of each of the small and medium.) This mouthpiece is essentially a 6 1/2 AL with a deeper cup. Compared to a 6 1/2 AL, it has smoother tone in the lower register, sings in the middle register, and retains stability and confidence in the upper register. It has the smaller throat, more similar to the 6 1/2 AL than the SM6 or 6BY, so if a 6 1/2 AL mouthpiece is what you are coming from, and like the breath support but want more euph tone, this is the mouthpiece to get.
The 6BY is the original Denis Wick euph or bari mouthpiece in that size for "USA" receivers, derived from their original trombone designs by enlarging the throat and backbore. It is an average mouthpiece. It is not popular, because most folks want the larger rim and cup from the 5 series or 4 series mouthpieces. For some folks, it still sounds a little "trombone-y," which is why Steven Mead got into designing mouthpieces.
Hope this helps. Link to Denis Wick mouthpiece chart:
https://www.deniswick.com/wp-content/up ... -Chart.pdf