I would really like to ask why? The bucking transformer lowers the voltage by a percentage placing your effective voltage in a lower range, but the variac will allow you a more specific setting according to the amp design. I just don't see the benefit over the variac except maybe in weight. If someone mistakingly turned off the bucking transformer you would jump back to what-ever voltage was present at the wall outlet. If someone mistakingly moved the variac dial to 100%, you would also just end up with the same wall voltage. So, what am I missing here?