While I can appreciate from a technical standpoint it must be frustrating to not have consistency across voices, I also would counsel against being overly concerned about it - there's a great Martin Carthy track on (I think) Shearwater where the song has quite a wide range and he uses three very distinct voices, without a hint of an attempt to worry about consistency. Feel and drama are infinitely more important in your singing than a technical discolouration few will notice and even fewer will understand.