I can’t speak for the Tuba, other than they don’t admin a lot of tuba students from what I know, they basically admit enough to fill the requirements of their orchestras.
I agee with compdad, probably the best thing you could do would be to seek out a high level teacher, preferably at a music school within a college, to see what they say.When you say you have been working without a teacher, did you mean you didn’t have a private teacher (ie you took lessons in school), or you taught yourself to play?
Tuba and other brass instruments are a bit different that let’s say strings, because it is very difficult to start those at a young age, so your relative lack of experience will be a lot less of an issue than it would be on strings. That said, though, Tuba players getting into music schools generally have worked with private teachers from what little I know, and given the nature of music teaching in school, it is going to be tough sledding. Biennen and USC music schools for a BM are some of the top music schools out there (lot of kids of conservatory level admission capability, I am talking the CIM/Juilliard/Eastman/NEC et al) go to places like Biennen and USC because they can double degree, so it takes serious talent to get in there. I am not as familiar with Gettysburgh, but the fact that it is an auditioned program says that it is likely not to be a place ‘anyone’ could get in, and even if it is not in the same league with the top schools in terms of admission, it still requires more than a fair amount of ability to get in.
It is why the evaluation is important, because that will guide you in what you do, and you do have options to think about:
1)You might be good enough to get into a decent program, with some polishing. The evaluation would tell you weak areas, and if it is just polishing it might be doable in 4 months or so (some programs it might be December 1st, if they have a prescreen, not sure if brass programs do that).
2)You could get into a BA program on Tuba, which generally is a bit easier than getting into a BM program, some BA programs are non auditioned. The advantage is you could do lessons with a teacher who is a tubist, and improve, and perhaps by the time you graduate you could think of doing an MM on Tuba.
3)If you thought of teaching, there is the Music Ed degree, which leads towards teaching music in the public schools. That though I would reserve only if you seriously want to teach in a public school, K-12, and I am not saying it is easier to get into than a BM program, Music Ed programs require competency on your core instrument at a pretty high level from what I know (others can chime in on that). Don’t see it as a fallback I recommend, only do this if you think you would like to teach music, there are far too many frustrated performers teaching music out there, and they don’t do their kids or music much of a benefit IMO.
4)You can also go to a good school that doesn’t have a music school per se, and still take lessons and do orchestra and band and ensembles. For example (and just that, not saying you want to go there or feel you can get in), the Ivies and other elite schools maintain high level music programs, they will pay for lessons, and have strong orchestra and other ensembles for non majors. You could find a school with such a program, get a degree in something, and use the experience there to boost your ability, maybe enough to then go for a BM.
Again, a lot of that depends on where you are right now, and no one on here, even someone who knows tuba, could evaluate your chances for admission to any of these options. It is why I agreed with compdad, the evaluation will tell you what your options are. The good news is everything in music is a reach, that symphony job is a reach for all people auditioning (because there aren’t a lot of those jobs, and they open up infrequently), so you reaching is not unusual, the bad news is that it is possible to be so far behind the curve of others playing the instrument that the dream of doing this professionally may be just that, a dream. On the other hand, that doesn’t mean you can’t do music, you could get a degree in something while playing, and then play in a variety of amateur or semi pro orchestras and bands that are out there, lot of people who are doctors and lawyers and investment bankers and the like play in groups like this, and many of them are nothing to sneeze at in their ability, lot of those people were talented music students who went the career route but still wanted to play music:)