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How does one know which key of tuba to use when playing pieces (orchestral or solo), is it by looking at the range and choosing the one that fits the range best or does it depend on the history of the piece/what the composer had in mind, and if it is that, how would I know which key to use?

I would say it's more important to think of what the composer intended when they wrote the piece. For example, Ravel would have been familiar with the french tuba when he orchestrated Pictures at an Exhibition. That partially explains why the Bydlo solo is so high. The predominant 6/4 CC tuba is not always most appropriate.

I definitely think it is important to consider the historical context, but I also heavily consider the tone quality required for the piece, at least to my minds ear.

For example, last season we did Cesar Frank’s Symphony in D major. I don’t have a French c tuba, but realize that the tone needed to be a little lighter, so I used a small F tuba. This next performance, we’re doing Kallinnikov’s 1st Symphony (which I’ve been asked to double the string bass for the first three movements), so I’m using a 6/4 CC. We’re also doing Shotakovich’s 5th Symphony with a large orchestra, so a 5/4 CC it will be. Did Berlioz Symphony Fantastique (5/4 rotary F), and Hungarian March (large piston 5/4 F) in the last few seasons.
This past weekend, I used a 5/4 piston F for Hindemith’s Symphonic Metamorphosis. . . Perhaps an unusual choice, but I wanted to ensure the clarity of the tuba part with this orchestra.
I have similar range on all the tubas, so it really comes down to tonal concept after considering the historical concept. YMMV

For me, range has little to do with it, unless the piece is fairly new and lacks much of a tradition, in which case I will use whatever is easiest for me to play. Otherwise, I try to study the scores (or I did when I was younger) and listen to very good recordings. I decide how "thick" or "thin" the scoring is. If it needs to be transparent I use an F. If the piece is really dense I use my BAT. Anything else I will default to a 4/4 CC. However, after the first rehearsal I have been known to dump a horn and show up for work the next day with one that seems like it will be more appropriate.

It is all a matter of taste and common sense; there are NO rules. You need to be able to play high and soft with control on a BAT and you need to be able to play fat and loud in the low range on an F. The key and range of a horn should not be a limitation - the weight and color of the sound the horn makes ought to inform your choice more than range. I have played Meistersinger on all three types over the years. I have done this on most pieces. My taste changes over time. What I want to sound like within the ensemble has changed a lot over the years.

You will find your way. My advice to you would be to not worry about it at all, ever. Unless you are in some sort of historically accurate group that uses period instruments (which is really uncommon) I can assure that NO ONE BUT TUBA PLAYERS CARE WHICH TUBA YOU USE, SO LONG AS YOU SHOW UP PREPARED, AND PLAY WITH EXCELLENT TIME, RHYTHM, AND PITCH.

Good luck!

Range of the instrument is not normally the concern. I could, within reason, play most any piece on most any tuba. Sometimes I choose which piece to play something on based on making the fingerings a little easier. I usually choose what tuba to play based on the sound I have in mind. Right now I only have access to a 5/4 CC (B&S PT6P) and a larger rotor F (Willson 3200s), but that is a pretty good setup. I could play everything I am working on right now on either tuba, but I don't. The CC is for large ensemble and moments in chamber/solo when a contrabass tuba sound is needed. The F is used for most of the other stuff. If you had access to a whole arsenal of tubas, you could really choose whichever horn you want based on a myriad of reasons.