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I've found lots more stuff written for a quartet than for a trio. You can always double a part if you choose to add trio literature to your books.

I started a tuba/euphonium quartet(we had 2 tubas and 2 euphoniums) in my school and honestly having gave the quartet such a wider range of pieces to play and it also gave the quartet a warmer sound so that is just a suggestion.
As for pieces i recommend having a quartet over a trio because you can play quartet songs and also be able play trio songs.
Good Luck

Dave Gannett(Tuba Gear) wrote some great trios in the 70's. We all played tubas, Dave on his 2J on the high parts , and the other two of us playing a King and a York. The parts would probably work best with an F or Eb on Dave's parts, a small C, and a good sized Bb.I don't think he ever published them but he might sell you a set of copies or something like that. On the Quartet side of this discussion, I played in the Tuba Quartet at the Disney Studios.They had some terrific players Gail Robertson, Chris Olka, and Willie Clark to name a few. There's quite a bit written for it. The thing that always puzzeled me was the horns we used. In a Sax Quartet there are four different horns. Usually Soprano, Alto Tenor, and Bari.Four Different sounds. Like a Barbershop Quartet. Most tuba quartets use two euphoniums and two tubas which, to my ear is Two Different sounds. I have always been of the opinion that it would have more range and blend better with an Alto, a Euphonium, an Eb, and a Bb.In fourths, like a Sax Quartet.I was in the process of putting one of these together last year, and the French Horn player who was going to play Alto actually did maybe 6 or 7 charts.I bailed when I came to the conclusion that the only place we would work was at theme parks and after 30 years of it I've had enough and only care to play the Orchestra at Epcot at Christmastime, and The Notorious Banjo Brothers and Bob me filling in as Bob. If anyone is interested in these charts I'd be happy to hook you up with John the French Horn playing arranger.Hope you all have some fun. Ed