The application of a voltage, V0, across a photorefractive crystal in the presence of detuning is well known as a means of enhancing two-wave gain. By mounting a crystal of bismuth silicate, with faces cut parallel to the (001), (110) and (110) planes, in a rotating holder, measurements of two wave gain as a function of (theta) , the angle between the electric field vector E0 (in the  direction) and the grating vector, K (in the (110) plane), have been made. Our results show that by having a non-zero value of (theta) the signal gain can be improved by a factor of two. Furthermore the experimental results showed that the (theta) dependent gain peaks differ in magnitude according to whether the crystal is rotated in an anti-clockwise or clockwise direction. In order to model the experimental results the steady-state expression for the space-charge field is extended to two dimensions and the usually neglected piezoelectric effect is included in the analysis of the modulation of the refractive index by the presence of this field inside the crystal. The asymmetry of the gain peaks is found to be caused by the optical activity of bismuth silicate and is predicted by the theory.