pianississimo: as I've reminded myself, so I'm reminding (possibly even helping...??) others as well

[bloke]

ppp - if marked in a (rare) tuba solo, might just actually mean "piano" p, whereby a particular composer may be accustomed to hearing tuba players play the soft passages in their compositions insensitively (read: way too loudly).

Maybe, we should first try just "p" in a (rare) solo passage. In an orchestra (if something marked "very-VERY soft" is - simply - played "softly"), a tuba player will likely receive a smile (assuming the solo passage is played well) from a music director.

Some band conductors - who might view loudness markings at some sorts of "decibel absolutes" (particularly if they are band tuba players) - might call someone out for only playing a solo passage (marked "ppp") only softly (rather than extremely softly)...and yes, soft tuba solos (played in wind bands) nearly always work out far better played by one person.

Playing a solo passage too soft to hear or (even worse) too soft to control never works out well.

All of that having been said...

If there is someone reading this who encounters difficulty playing extremely softly, the trick is to rely on more things than just "air/air speed" - or whatever the "air-is-everthing" crowd label it. The EMBOUCHURE (changing it for varying loudnesses) plays a key role in controlling response and pitch at volume extremes. These are words typed into a textbox, so I'm not going to get any more specific than this. If someone (who has trouble with control at volume extremes) is reading this, TRY various embouchure things. I wager such people will eventually stumble upon success, and will enjoy having mastered yet more playing skills.
one last thing: Once soft/very soft/etc. playing is mastered, the nervousness (often associated with it) evaporates, which defines that it is then executed even better.

- 'fesser bloke

[MikeMason]

Last time I played Mahler 1 I could never get the solo soft enough on a pt16 f to please the conductor. Between rehearsals I borrowed a euphonium, spent about 10 minutes acquainting myself. Conductor smiled and nodded on next time through,and that was settled. Happy employer = successful gig.

[bloke]

The only thing I’m playing extremely softly is the sustained D (as well as the sustained double low F in the first movement) - because they’re relatively (very) insignificant in the texture, and so I can sustain them for a long time.
Having played the solo passage in three of the four rehearsals (whereby one rehearsal only hit Mahler spots needing attention, and concentrated more on the other two pieces), the music director said not a word about it.

[bloke]

Being redundant here...

Solo notes/pairs of notes/phrases/passages need to be heard (and still prominent) even if played softly.
"Soft" is just as much an expression marking as it is a quasi decibel marking.
Many music directors "get" this...a few do not.

"Crazy soft" (in the context of symphony orchestras or bands) is when the strings (or clarinets - as this is something that strings and clarinets do particularly well) enter in such a way that the patrons can't be certain whether anyone has actually begun playing (ie. a special effect - and mostly in the context of the players I just specifically identified).

me...??
I very talented young trombonist complimented me (both after the dress and after the evening concert) on the little solo passage.
I've hooked them up with a few gigs...LOL...It's hard to know whether they were sucking up. I generally accept compliments graciously, and never take them particularly seriously.
noted: The bass and bassoon were given bows. I was not. ' probably the same again this afternoon.