I am working on your standard SE 300B With DC heating and Potentiometer controlled input. It is very quiet at lower volume. As you turn it up it has a hum that gets louder and louder as you increase volume. Is this normal? It is quiet in actual use with 96db speakers. Using it with a pre amp means turning the 300b up just enough to get a good volume and using the pre for the rest of the amplification.