The Berlin Brass expansion A has the same excellent qualities as the main library, so if you have that the sonic quality is just as great.
The Euphonium is the only instrument I've really toyed with as I don't tend to need the extended family of instruments for what I write; standard orchestra is more than enough!. I do think it is wonderful for a warm colour alongside the Berlin Brass Trombones.
This Bass Trumpet could cover the range of a Trombone as a substitute, except for the lowest notes (but you can extend them in Capsule). The tone is brighter than that of the Trombones in the main library and it retains a trumpet's edge, even at lower dynamics.
If you don't own the main library, Expansion A (and Expansion B for some very deep mutes) are also good ways to get your feet wet on how the main library sounds; I got them in the last Completion Days sales.