Take a look at Brown's "Application Advice" page here: https://www.brown.edu/academics/philosophy/application-advice
Their advice under LoR isn't bad -- quite understandable -- but still, it makes me cringe a bit; because, well, it seems a bit fake: to intentionally foster some sort of close relationship with professors for a year or so, whom you know you will be very likely to ask for those letters when you apply for PhD.
I'm on the same boat -- sort of, Bachelor in my case. I'm (very) close with my supervisor; she knows me and my work well, and I was in all her classes throughout undergrad; so that's good. I have taken one course with another philosophy professor; he gave me good grades for both of my term papers, but we never really talked and it has been almost a year; still, I think he'll remember me (I came up with my own essay questions and asked him about them -- I think very few, if any, in my class did that), and I'm planning to send him those papers to refresh his memory after I meet with him (soon). I'm still not sure whom I'm going to ask for the final letter, trying to decide between (1) one of my thesis markers, whose class I have never attended to and I don't think she even knows who I am, and (2) yet another philosophy professor, whose Level 2 (out of 3) class I have attended to and did OK in, and with whom I'm currently working as an assistant for a quasi-philosophy course. I'm more inclined towards (2) at this point, since he has a more comprehensive sense of who I am as a person and a bit of knowledge on my philosophical attitude/work.
@Glasperlenspieler: any thoughts?
I don't think you are over-stressing: some say LoRs are even more important than the GRE score, because they are a much better indication of your ability to do well in philosophy. That being said, remember that SoP and writing sample are two equally important components you have full control of!