Hi XR_Racer and welcome to the forum

!
I must confess I'm a little confused about the background to this issue (specifically the source material), but I'll make a "stab in the dark" anyway.
As John says, the lowest note on a bass tuba (and in the library) is a low Bb (MIDI note 22) He is also correct, of course, in saying that the sample can be stretched below that. Before you make your score impossible to play, however, (and/or make your "virtual tuba player" sound like he's suffering from extreme flatulence

) perhaps I could venture an explanation for the issue that you appear to be experiencing in Finale.
The lowest
written note for a tuba part is the Bb below the bass clef staff (nominally MIDI note 34). The tuba, however, is a
transposing instrument that sounds an octave below the notation (so your "low Bb" actually
sounds an octave below that - hence MIDI note 22). Unfortunately, one of Finale's "endearing little quirks" is that it honours transposition during playback
but not during editing! This means that you'll actually hear the note you've
written rather than the one you want
played! As if that wasn't enough chaos to deal with, Finale also applies the "out of range test" to the
written notes rather than the
sounding notes - so you won't get the orange note head indicator until you
write notes below the lowest
sounding notes! Apparently this long standing bug ("feature request"?

) is scheduled to be corrected in the next version but when that's due is anybody's guess ...
I hope I've got the right end of the stick here and that you can get enough sense out of the above to inform your progress (if not then "as you were"

). Right now, however, I'm afraid I need a nap - my brain hurts

!
Kind regards,
Keith