The purpose of this effort is to investigate the effect of bias conditions on the power harvested using magnetostrictive materials. Towards that end, we first develop an analytical model to describe the dependence of the constitutive parameters on the bias conditions. We validate this model experimentally and define a range for its validity. We obtain a one-dimensional lumped-parameter model of the energy harvester and optimize it with respect to the load resistance and frequency ratio. The optimal expressions are then used to study the effect of bias conditions on the optimal values. We observe that the bias conditions significantly affect the antiresonance frequency allowing for possible real time control to maximize the energy flow from the environment to the load. Furthermore, it is observed that, in the range of bias conditions for which our model is valid, the harvested power increases with magnetic bias and decreases with the prestress.