The sub-glass-transition viscoelastic and physical aging responses of an amorphous poly(ethylene naphthalate) (PEN) have been studied using uniaxial tension stress relaxation experiments. It is known that PEN exhibits a strong β relaxation that overlaps the α relaxation in the experimental time and temperature ranges studied. In prior work, we had shown that both amorphous and semicrystalline PEN exhibit thermorheologically complex behaviors in that neither time-temperature nor time-aging time superposition apply to the materials. Here we compare results from two-step aging experiments in which the material is first annealed at a temperature near the nominal glass transition temperature of 120 °C. In the second step, the viscoelastic response of the material is tested at a lower temperature, following classical sequential aging techniques. We find, for samples first annealed at 100 °C, that the amorphous PEN shows time-temperature superposition behavior for constant annealing times. The results are interpreted in terms of an isostructural or constant fictive temperature glass. When the responses are compared isothermally for different fictive temperatures, as found previously, thermorheological simplicity breaks down. Possible reasons for this behavior are discussed.