This sounds like an internal layout problem to me. It is true however that with an open input (nothing connected) that there will be a greater amount of noise present. But that should be gone with your preamp connected.
Now one exception may be that your interconnect cable is responsible for some of the buzz. So it is a good idea to try a different cable before blaming the amp!
Now one exception may be that your interconnect cable is responsible for some of the buzz. So it is a good idea to try a different cable before blaming the amp!