The similarities between the 339 and 336 are that both have a centerblock and are archtops. The 336 has solid woods and carved mahogany and the 335/339 have laminates. The Midtown has a centerblock and solid woods (maple top and Mahogany back and sides), with a flat top. These are 24.75" scale.
The JohnnyA model has solid woods of a Maple top and Carved mahogany back with an archtop. It's main difference from the other models is that it has a hollowbody and 25.5" scale.
The presence of the centerblock tends to reduce feedback. But the relative thinness of the Johnny A tends to buck feedback. I lust for a JohnnyA guitar, and this is my excuse for knowing this level of minutiae.