Colleges will not help you pay off your undergraduate student loans, and you should NEVER go for a PhD without full financial support, especially for a field like Music theory which has few jobs, even fewer tenure track jobs, and even those generally do not pay very well.
No reputable PhD program will accept a graduate student without providing at least five years of full tuition and a research or teaching assistantship. If they do not have the funding they will not accept any new students.
In general, the best advice that I can give anybody who wants to be a music theory professor is “don’t”.
Like most of the humanities, music theory is both obsessed with prestige as well as having very few open positions. What this means is that the vast majority of tenure track academic positions in most humanities are filled by graduates of the most prestigious colleges, because there are simply so few that any college can have their pick from the graduates of almost any PhD program out there.
Unfortunately, unless you finish your undergraduate degree at a “prestigious” college, your chances at being accepted to a “prestigious” PhD program are very low. If you transfer to a well known college to finish your degree, your chances will increase, however, they still will not be high.
What is worse is that the at least 2/3 of all jobs in music theory are not tenure-track and not even full time positions, but adjunct positions, meaning that the instructors are hired on a semester basis, by course.
The journey to ending up with a PhD is long a difficult, but it is even more difficult to get a tenure track position in music theory. Your chances at ending up as a professor at a college are slim at best.
This past year there were about 27 tenure track positions advertised in Music Theory. That is fewer than the number of PhD students who graduate from the top PhD programs every year.
If you want to end up with a PhD in music theory, go for it. However, you should not plan on ending up a professor in Music theory.