Binaural rendering is central to spatial audio reproduction via headphones and wearable devices, yet systematic evaluation of enhancement techniques remains methodologically inconsistent across the literature. This paper presents and applies a subjective evaluation methodology designed to consistently elicit five perceptual attributes of headphone-based spatial audio: externalization, elevation fidelity, front/back distinction, room tone, and spectral coloration. The methodology combines absolute judgment and relative comParison protocols, taking advantage of their complementary capabilities to capture both the absolute perceptual quality of individual rendering conditions, and the salience of perceived differences between them. It is applied in a controlled experiment comparing conventional HRTF-based binaural rendering against two enhancement variants that each superpose a masked spatially diffuse sound field component into the binaural output. Stimuli were spatialized across eleven azimuthal positions in the horizontal plane using a generic dummy-head dataset, and presented over closed-back headphones to participants. The results validate the proposed methodology as a tool for revealing perceptually relevant differences among binaural rendering conditions. Relative comParison tests reveal additional performance difference details between rendering methods. Both enhancement methods significantly improve externalization and front/back distinction relative to unenhanced binaural rendering, with the largest gains at lateral azimuths, and without a statistically significant increase in perceived spectral coloration beyond the baseline effect of HRTF filtering. However, the results do not indicate conclusively whether the binaural rendering methods examined here exhibit or mitigate a "spurious elevation" artifact associated with frontal sound presentation.
