This is a really interesting conversation. I guess I never really thought about it because I'm a euphoniumist first, then a trombonist, then a tubist.
Since everyone has given some great responses, I'll give my $0.002
Tuba: The tuba is the youngest and lowest brass instrument. Don't be scared of this awesome instrument. The tuba will round out the bottom end of your music so well.
It can project as well be an additional foundation to your bass.
Euphonium: Not a very common orchestra instrument. VERY common in band. Some people use this in when parts are written for tenor tuba. This instrument is the closest to the male voice (some argue trombone.) Even with that argument, it's more agile than the trombone because of valves.
Trumpets and Trombones: Man, what a duo. VERY powerful instruments with much punch.. YOu can use them in so many ways. You don't have to relegate these awesome instruments to just fanfare type motifs. They are very capable of handle rigorous AND MELODIOUSLY warm passages. Don't look at stuff in the classical period as your guide for these instruments. The trumpets were not valved, thus very limited. Also, IN THAT TIME, people didn't realize the trombone could be a virtuous instrument.
Horns: IMO, there is no better instrument that can "bring home the bacon" like the French Horn. There's nothing like giving the Horn a countermelody in ANY range and you feel it.
I leave you with the GREAT RESPIGHI!