In previous work, a phenomenologically constitutive model was presented describing the finite, nonlinear, viscoelastic behaviour of polymer glasses up to yield. This model was, however, restricted to thermorheologically simple materials. In this paper this restriction is removed, thus extending the model to materials behaving thermorheologically complex. Based on linear viscoelasticity, this extension can be achieved by either adding a process in parallel, or in series. Experiments in the plastic range suggested an approach based on stress additivity, i.e. two processes in parallel. The resulting model consists of two linear relaxation time spectra in parallel, each having its own characteristic stress and temperature dependence. Whereas in the case of a single process the influence of stress and temperature is comparable, this is no longer valid for two processes since the molecular processes depend on a part of the applied stress rather than on the total applied stress itself. Numerical predictions using the extended representation showed that the model correctly describes the yield behaviour observed in practice. Simulations of creep experiments at various stress levels and temperatures showed a good qualitative agreement with experimental observations in literature.