This paper examines the problem of designing fixed-rate transform coders for sources with arbitrary distributions, under input-weighted squared error distortion measures. As a component of this system, a flexible scalar compander using Gaussian mixtures is proposed. An algorithm is developed to set the parameters of the system using a data-driven technique that automatically balances the source statistics, distortion measure, and structure of the transform coder to minimize the high-rate distortion. The implementation of Gaussian mixture companders is explored, resulting in a flexible, low-complexity scalar quantizer. The operation of this system for the problem of wideband speech spectrum quantization with log spectral distortion is illustrated, and shown to provide good performance with very low, rate-independent complexity