I think it's best to get the second opinion from a different medical institution. In the Boston area many of the hospitals are affiliated with Harvard, but there are some differences between, say Beth Israel Deaconess, and MGH. I've had excellent results at Beth Israel, as well as Brigham and women's. Lots of "groupthink" that goes on in hospitals and even something so simple as a way a procedure is done can make a difference in the diagnosis.
In the past I have gotten up the food chain, to the surgeons or other specialists, by starting with a generalized doctor in the department. Your PCP should be able to make references for you, at least in his or her medical institution, but they don't always do so or do so very well. In the end, you need to be your own advocate. It sounds like you are doing a good job of advocating for yourself, but I thought I'd add my two cents
Good luck.