I would like to chime in. I played trombone, euphonium, and tuba in high school. Since then I have played all of them separately.
Do you have access to play different trombones? Have you played a trombone?
What model euphonium do you play? If it is a large bore compensating euphonium I recommend you play a .547" bore tenor trombone with an F-attachment (it takes a large bore mouthpiece). You can even keep the same mouthpiece. I played bass parts in the orchestra with a similar horn with the advantage of also being able to play the second and first parts if needed. I could also play in jazz, rock, and concert groups with it.
Bass trombone is a specialty which I found better doubled with tuba. When you get into bass trombone you have many more (expensive) options concerning single valve, double vale, configuration, etc. I do not know your budget.
Good luck with your choice.
Cerveny BBb Kaiser Tuba
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