You said that you would be applying for doctoral programs for Fall '18. If you started your current master's in Fall '13, and you will apply to doctoral programs for Fall '18, that is a 5-year span (F13-S14, F14-S15, F15-S16, F16-S17, F17-S18, then F18).
You have said yourself that you "suppose there isn't really a great benefit from doing a second master's," and I agree. I could understand pursuing a second master's because you changed your mind about what it is you want to study, but you're talking about completely changing fields temporarily (and I assume you have learned by now that graduate programs are much more field-oriented than undergraduate programs; please correct me if I am wrong) for the sake of improving your English and securing more letter-writers. Guitar performance is completely different from music theory, and I can't imagine applying to a program without the genuine intention to focus on that field afterword.
I also agree with music that you may be looking at references the wrong way, and unless you are performing poorly at your current program, you must be undervaluing the faculty. The quality of your scholarship is absolutely the most important aspect of your academic profile.