I find that the Wick 3AL works best for me in this capacity. For example, it allows me to play both euphonium and tuba parts. The 2AL is just a bit too big, and I've never been able to get a Schilke 60 to play in tune on a euphonium (too deep, I think).
If you want to play "real" euphonium parts with it, you may want to go a little smaller than the 3AL. My Doug Elliott mouthpiece is just a bit smaller than a 3AL and I use it when playing euphonium parts that require a lot of high register work.
I have not found the SM Wick mouthpieces to my liking. But you may have a different experience.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)