Recent developments in wave energy converter (WEC) control include robust control strategies, which admit a level of uncertainty in the model description. Then, for the application of such control methods, some knowledge of the uncertainty bounds is initially required. As recently shown in the literature, some approaches for uncertainty quantification can lead to scenarios where, due to uncertainty quantification, the passivity of the WEC model is not guaranteed and, consequently, the optimisation required to capture most of the available energy cannot be successfully performed. Such approaches can generate significant performance losses. Thus, in this study, a passivisation methodology to extend the applicability of spectral control approaches is presented. The benefit of the presented approach, relative to previous results, is shown with an application case.