We study the influence of vectorial wave coupling on the wave eigenmodes in an arbitrarily-cut photorefractive and optically active piezo-electric sillenite cry stal, and solve the vectorial equations for weak two-wave coupling. We deal exac tly with the vectorial wave coupling in an optically inactive piezo-electric cry stal by solving the scalar equations. By doing so we calculate the signal gains in the optically active and the optically inactive crystals and discuss the cont ributions of the isotropic and the anisotropic coupling to the gain, respectivel y. We optimize the signal gain of the optically active crystal and apply the re sults to a BSO and BTO crystal. We also investigate the influence of the piezo-e lectric and elasto-optic effect on the optimum gain for (110),(111),(112) and th eir equivalent cuts.