For many tuba players, something like the Willson/Marzan 2975 seems a good idea since it and the Wessex Festivo both have lean to the left/front action compensating designs. I personally often play tuba parts in pitch in community band on either my Adams E3 or my Sterling Virtuoso using a Bach 1.5G bass trombone mouthpiece.
I know that Mike Roylance of the BSO plays the euphonium parts on Planets, Mahler #?, and a couple of other symphonic pieces using a Yamaha 642 (don't know the mouthpiece). He hires a freelance tuba player most of the time instead of a euphonium player.
Doug
Adams E3 0.60 Sterling bell - Prototype top sprung valves
Concord Band
Winchendon Winds
Townsend Military Band