Yes that (the focus on tranquility / ataraxia / peace of mind as some unique kind of highest pleasure) is a common assertion that I reject, Mike, and I think you will find that Dewitt states it considerably differently. In fact I do not believe that either ataraxia or aponia are "kinds of pleasure." I believe they are adverbs that describe ways / contexts in which pleasure (ordinary pleasures of all kind) are experienced. In other words, the best way to experience any pleasure is "without distraction" (ataraxia) and "without pain" (aponia).
For the record, this is Cassius' view and is not shared by all. The sources that use ataraxia include Letter to Menoeceus:
QuoteThe steady contemplation of these facts enables you to understand everything that you accept or reject in terms of the health of the body and the serenity of the soul — since that is the goal of a completely happy life.
τούτων γὰρ ἀπλανὴς θεωρία πᾶσαν αἵρεσιν καὶ φυγὴν ἐπανάγειν οἶδεν ἐπὶ τὴν τοῦ σώματος ὑγίειαν καὶ τὴν τῆς ψυχῆς ἀταραξίαν, ἐπεὶ τοῦτο τοῦ μακαρίως ζῆν ἐστι τέλος
(where ἀταραξίαν/ataraxian is translated as "serenity of the soul"). And so this term is used in LMenoeceus by Epicurus, where it is offered as a criterion for choices and avoidances.
Also, in Diogenes' Wall, we find this, where we are able to contrast ataraxia versus the ills of the soul that it's meant to heal: the perturbances of the soul:
https://theautarkist.wordpress.com/2017/03/25/dio…-the-pleasures/
QuoteLet us now [investigate] how life is to be made pleasant for us both in states and in actions.
Let us first discuss states, keeping an eye on the point that, when the emotions which disturb the soul are removed, those which produce pleasure enter into it to take their place.
Well, what are the disturbing emotions? [They are] fears —of the gods, of death, and of [pains]— and, besides [these], desires that [outrun] the limits fixed by nature. These are the roots of all evils, and, [unless] we cut them off, [a multitude] of evils will grow [upon] us.