Something I tend to see quite often (with those who actually/occasionally discuss playing the tuba, and not exclusively instruments, bags, other accessories, and tuba personalities) is a distinction between "bel canto" etudes (melodic - studying phrasing and musical expression) and "technical" etudes.
When discussing "technical" etudes, players will discuss which ones they prefer (championing this-or-that "technical" book, as they perceive them as being more "interesting" than others...(likely: because they perceive their favorite{s} as more "bel canto" oriented).
If there's a player who (over-and-over - decade-in/decade-out) has continued to revisit the Bordogni 40-whatever etudes (mostly from Rochut book 1) or has gone through Rochut book 1 over-and-over, or Shoemaker (etc...)
Consider digging into Rochut books 2 and 3.
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>> These subsequent Rochut-trombone/Bordogni books become a good bit more "technical" while continuing to be "bel canto" etudes. (ie. they become both "technical" yet continue being "bel canto" etudes...and - hey - if a player has not studied them before - yes: they still are clearly identifiable as Bordogni, but no: they are NOT the "same old $h!t"...and (yup) they're challenging. Quite a few of them are 2-1/2 to 4 pages long, so there's an "endurance" factor in the subsequent books, as well. (Not lower, but) the upper range - in the subsequent books - is also extended.
Many players like to play these two octaves lower than Rochut's trombone octave. That's really good for "working" the low range, but (seriously) when doing that they - pretty much - become more "technical" and less "bel canto" (as - necessarily - the overwhelming majority of phrases will need to be broken - coming up for air - than when playing them one octave below the trombone octave, which is already going to require more air and more phrase "rethinking" than when playing them on trombone (or sung by a soprano).
Particularly with (as an "old man") this
- 6/4
- extra-large bore
- rotary
(all three of which offer MORE challenges - regarding achieving a pleasing "legato")
tuba, playing through all the etudes (with their super-long phrasing marks) and playing them AS IF they are "slur" markings (which they mostly are not), is quite challenging.
Personally (as now: recovering from a lung infection) I've had to RE-BREAK an old long-ago-broken bad habit - a habit of "huffing" through legatos (rather than smoothly blowing through them). More personal (not-important-to-anyone-reading-this) information, is that I'm reminding myself to take in generous amounts of air (as it no longer triggers coughing) as well as the "smooth air" (vs. huh-huh-huh thing, which does nothing good, limits speed, destroys phrases and - well - sounds bad).
When I decide to study any of them with more of a realistic variety of articulations, I'll scan/print out a selected etude and edit it myself, including (to me) musically-appropriate articulations.
Considering "technical" (which typically involve a lot of "tonguing") etudes vs. "bel canto" etudes, think through quintet/orchestral/band (ok, and - doesn't exist in the free market) and solo works...a considerably larger percentage of passages (particularly the passages which we judge as a bit "treacherous") are legato, whereby "wham-wham-wham" or even "whammity-whammity-whammity-wham" passages tend to not be as difficult.
One thing (as I'm grateful to see them, as my own playing certainly needs work on these) fairly common in the later Rochut books' Bordogni passages are strings of arpeggiated pitches (from high) which work their way (supposedly) gracefully to low.
OK...This isn't as important a skill, but...
Reading the bass clef at the octave (as well as being able to read C treble clef and B-flat treble clef) are all things that tuba players (ok..."working" tuba players or "tuba players who hope - someday - to work") should be able to read just as fluently as at-pitch bass clef.
summary: Rochut (trombone) Bordogni books 2 and 3...If only having brushed against them years ago (or - I'd wager as more typical: never having dived into them), and STUDY 'em...you know: "etudes".