In this work we analyse the complex amplitude response of volume holograms. This analysis suggests a new scheme for encoding complex amplitude wavefront information onto an optical hologram which may prove useful for combining computer-generated holograms (CGH) onto volume holograms. Specifically, we analyse the amplitude and phase modulation associated with a volume phase unslanted transmission grating. Using the expressions given by the Kogelnik's coupled wave theory (KCWT) we find that there is a coupling between the amplitude and the phase modulations. This coupling can be controlled to some extent by means of the reconstruction angle: we have found that the significant magnitude is the normalized Bragg detune angle. A large variation range of the grating strength is desirable, which is enhanced by the use of shorter wavelengths, and materials exhibiting overmodulation capability and/or with a high thickness.