The perceptual evaluation of the Audio Augmented Reality (AAR) experience is typically conducted using authenticity or plausibility as a measure of realism of the sounds. The previous studies usually employed methodologies where participants were comparing the exact reference with real sound. This paper proposes a novel experimental design where participants rate the plausibility of a pair of real or virtual loudspeakers playing consecutively from different positions in the room. This approach sets expectations of the user very close to real-life scenarios where similar but not identical sources exist within one environment. The objective of the study is to assess plausibility and to evaluate spatial sound attributes using two auralization methods which simulate the experimental room. Instead of using the yes/no paradigm, the proposed methodology employs continuous scales which allow gaining insight into the correlation of plausibility with other sound attributes. The preliminary results show that the proposed experimental design was successful in obtaining a meaningful comparison between the two auralizations and the real source. Moreover, the analysis of results suggests that plausibility is a continuous dimension.