The MPEG-4 Scalable to Lossless (SLS) audio coding is recently being developed to provide a unified solution for high - compression perceptual audio coding and high-quality lossless audio coding. SLS provides efficient Fine Granular Scalable (FGS) coding from AAC core layer to lossless, and achieves reasonable perceptual quality at its scalable coding range using a sequential bit-plane scanning method, which minimizes the audio distortion according to the spectral shape of the core layer quantization errors. In this paper, it is shown that the perceptual quality performanc of SLS at intermediate rates can be further improved by incorporating psychoacoustic model into the bit-plane coding process. In addition, it is also found that such an improvement can be achieved by slightly tweaking the original bit-plane coding process of SLS and hence preserving its nice features such as compatibility to lossless coding and low complexity.