I would take that music back to wherever you got it.
If the tuba part really has only two flats, it apparently has been transposed for an F instrument. That would be extremely unusual.
Bb trombone is not normally written in a transposed key like the Bb trumpet (it happens, but if I remember right, the tuba would also be treble clef in that system?)
In by far the most common notational convention, the answer to your question would be "a tuba" - tuba players normally read concert pitch, regardless of the natural key of their instrument.
The stupid thing about your question is the topic question "Can you answer my question?" This invites people like me to look at it, who don't know anything but think they can answer all kinds of questions, but fails to pull in busy individuals who might recognize that they have specific expertise in the matter. Take some time to think up a relevant title for your questions.
OK. This is actually an easy question to answer, so I may be qualified to answer it.
In the band, certain instruments generally are referred to as "concert pitch." This indicates that they sound the actual pitch they read(yes, this implies some don't which I am coming to). Concert pitch instrument include the flute, oboe, bassoon, trombone, baritone or euphonium(when reading bass clef), tuba, and various pitched percussion instruments.
Certain other instruments are referred to as "transposing instruments." These instruments read music which is actually written in a key different from the pitches that are sounded. These instruments include the Eb clarinet, Bb clarinet Eb alto clarinet, Bb bass clarinet, Eb contrabass clarinet, Bb contrabass clarinet, Eb alto saxophone, Bb tenor saxophone, Eb baritone saxophone, Bb trumpet, and F horn.
All of these instrument, when playing their "C" are sound the name of their instrument. Ironically, going back to the group of concert pitch instruments, the trombone is often called a Bb trombone, but is actually a concert pitch instrument. Finally, our tubas are usually one of four varieties: BBb, CC, Eb, and F, however, all tubas are concert pitch instruments.
I'll let someone else deal with the British brass band tradition. I don't want to confuse the issue, here. Also, I did not list the piccolo, which sound exactly one octave higher than notated, and the string bass, which sound one octave lower.
It is likely I have left some other detail out, so those of you who catch it, please help to OP out.
It sounds to me that you have Brass Band parts (assuming the trombone is in treble clef also).
If so, the trombone part would be written in Bb transposed treble clef.
The bass part is written in the American (or ROW) custom of untransposed bass clef.
You simply need a tuba player that is used to reading that notation.
They are used to transposing on sight for their instrument.
Americans think of it as the Eb, CC, and BBb tubas all having different "fingerings".
Orchestral music is usually written using this convention anywhere.
If you're on this side of the pond, though, you need a trombone player who can read transposed treble clef.
Instead of talking to your plants, if you yelled at them would they still grow, but only to be troubled and insecure?
In the brass band tradition, all of the instruments were originally transposed for whatever key they were in to treble clef so that one could move from one instrument to another without any reading impact at all.
In the US, tradition became that trombone and tuba read concert pitch bass clef parts.
Orchestral music, as I have indicated, also follows this convention, with a few exceptions that you won't need to worry about.
I understand that there are more British brass band bass players that read bass clef now, and that parts are often printed with both versions to widen their coverage.
All that you will need to do is ask the if the tuba player reads bass clef. This bass clef part is untransposed, and if the tuba player plays bass clef it will be untransposed, so there will be no problem.
Valve trombone and slide trombone should be interchangeable. They just used valves if no slide player was available, and needed to convert someone else.
Instead of talking to your plants, if you yelled at them would they still grow, but only to be troubled and insecure?
Unaccustomed as I am to answering a 10-cent question with 10-dollar bill, I nevertheless can't resist the temptation to offer some historical perspective on the why.
Before the invention of valves, brass instruments were limited to the harmonic series of their open bugles. To play Telemann's Trumpet Concert in D, they needed to have a natural trumpet in D. To play music in C, they'd need a C trumpet. To make it easier for them to read music, all the music was written the same way, so that they adjusted the instrument to the music.
Trombones didn't suffer this limiation, because they had a slide. So, their music was written in the bass clef as it sounded.
In the quest for a chromatic scale, valves were eventually invented. That subsequently made the tuba possible, because valves allowed the player to play low in the harmonic series, where the open harmonics don't provide many note choices. Because the instrument was invented to be chromatic, the music written for it was written as sounded, in concert pitch, just like a trombone. Treble-clef instruments continued as they did before, and now if a trumpet player uses a C trumpet to play a Bb part, they must transpose.
Brass bands in the modern British sense came into being much later. They preferred a system of notation that would provide the same written music and fingerings no matter what instrument was being played, to accommodate switching around among the players. That's why all British brass band music is written in treble clef.
Since tubas come in different pitches, it is up to the player to press the right buttons for each pitch to be played.
I once played a church gig with a quintet, and the organist provided music for us to play. She looked up "tuba" and saw that it was a Bb instrument, and provided a part transposed from concert pitch (C) to Bb. I mentioned that in the future, she should write the music as it for organ, and she became angry, and thundered, "Isn't it a Bb instrument?"
"Actually, this one is an F."
"So, how am I supposed to know that you use an F tuba, which I didn't even know existed?"
"That's why you write the part in concert pitch, so that I can make the choice of which instrument to use." Then the light bulb went on.
Rick "never underestimating the musical territoriality of some church organists" Denney
Unaccustomed as I may be to taking on one of Rick Denney's erudite posts, I wonder if concert pitch tuba notation might actually predate the tuba, having been written for ophicleide - also a chromatic instrument, though for all I know the instruments of the time may have had some limitations in that respect.
I can only write from my own experience in playing in wind bands and brass band in Belgium.
I play the euphonium and BBb tuba.
I was taught (and this is common practice here) that the "C" (we call it "do") is to be found one line below the staff (in treble clef) and is to be played open. So it will sound like Bb (si-b to me) in concert pitch.
The same note can be found in bass clef-parts between the 2nd and 3rd line.
I get to play both treble clef and bass clef, in about 50-50%.
In our system, if I learn to play CC tuba: The C will still be played open, but will sound like C in concert pitch.
So we do not learn to use different fingerings, but we learn to get used to the fact that the C sounds differently when playing BBb or CC tuba.
Conclusion, it's like this:
When CC and BBb and EEb players are playing the same notes to a listener, they will be reading three different (transposed) parts.
If transposed parts are not available, we are taught to transpose on sight (so shift the C up or down the staff.)
I know It sounds a bit confusing when I read this again, but it is correct. Ask me if you don't understand my explanation. It's just another way of thinking, but it works ok.
I think in symphonic orchestras in Belgium, the 'american'-system is used.
