Re: transpose treble clef key of to bass clef for a b flat trombone
I play trombone, and there's a whole lot of us who rue the day they invented tenor clef, lol. Personally, I'd much rather see it scored an octave lower with an "8va" notation showing it's meant to be played an octave higher.
I think it's more about exposure. I have no desire to boast though I know it's going to sound like I am: I too play 'bone, however I first learned in a British tradition brass band where you play transposed treble clef - that's almost the same as tenor, just with 2 extra sharps... It was nearly 30 years before I was exposed to bass clef. And what about fake books? They're nearly always concert treble (yes, I know you can buy C, Bb, Eb and bass clef books but usually you only see concert treble).
It really didn't take much effort and these days I play transposed treble, concert treble, concert tenor and concert bass fluently. Very occasionally I forget what clef I'm in and make some interesting sounds, but mostly it's no problem. The situation I find most troublesome is clef changes in the middle of a part - I'd prefer to not have this, but again it's exposure. If the things I was playing had these changes more often I'd get used to it and the problem would go away.
I will say that the way I learned makes for some interesting things going on in my head. Take a second space "C" (bass clef). Because of the way I learned that 6th position "C" is really a "D" for me, in the same way a 1st position "Bb" is "C" and a 4th position "G" is really an "A" in my book. Better still, a 4th position "B" is actually a C# to me...
My advice is: "Don't fight it, instead 'go with the flow' and you will be surprised how much easier it becomes."
BTW, I regularly play in situations where I encounter Transposed Treble (a brass band) which is almost the same as Concert Tenor, Concert Treble (Jazz "real books" and some of the stuff I play at church) and Concert Bass (mostly big band charts and musicals {where I also sometimes see Concert Tenor} and anything where I've written a part for church).
And just to make it interesting, occasionally I've written something for church in bass clef and decided it works better on my trumpet - I don't bother rewriting it, I just play my trumpet from the bass clef part... Or sometimes it's the other way around - a trumpet part I've written might work better on my 'bone or Eupho - I don't bother with a rewrite - just play it. A little effort and it becomes easy.
Again, don't fight it, embrace it.