Pleasure and Pain as the Only Two Categories of Feeling, and the Issue of their "Blending"

The issue of whether pleasure and pain can "blend" together has come up in another thread. I am not prepared at the moment to post at length on the issue, but my understanding is that in Philebus (and perhaps elsewhere) the argument that pain and pleasure can "blend" together is suggested by Plato to be an argument for why Pleasure cannot be the guide / goal of life. If I recall, the issue is something to the effect that if they blend together to form a third category of feeling, and if reason is needed to identify the components, separate them, and make decisions about them, then REASON becomes the ultimate arbiter of things that are desirable and undesirable, and therefore Pleasure and Pain are dethroned. I may have the argument partly or completely wrong, but I remember it being an important argument that also relates to PURITY of pleasure. I am posting this here to mark the spot for a discussion of this topic when someone has the time to unwind Plato on it.