I agree with Dlwask. It all depends on the quality of power coming into your house and the quality of internal circuits. I could not hear any difference when I tried the 400 Pro in my system, but I have dedicated lines and plug staight into the wall. Others do find a difference and I don't doubt them or their ears. But I suspect many of these would find far less, if any, difference if they had dedicated lines. It is definitely the first place to start with power issues, and actually usually costs about half what a RG 400 Pro does.