I've done a Tuba Christmas kind of thing with mixed trombones and tubas (trombones playing the euph parts). It sounded, ... well ... "acceptable" but a little odd. And it's not just the sound. It's the articulation as well. I'd consider it for an "informal" setting, but it would otherwise be better to have euphs play the euph parts. This isn't to say that trombone and tuba don't sound good together. I think that a bass trombone and tuba combination can sound very good and interesting in some circumstances -- e.g., some of the Walter Sear Advanced Duets for Tuba. But in cases where that's true, it's also often true that the range is quite challenging for the trombone.
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)