I have one of these and cannot detect any change at all. It's between my preamp and amp. I have 4 different sets of tubes and swapping them still makes zero audible change. On a whim I bought the cheapo FX Audio buffer/pre and I can actually hear the difference. Any thiughts on why? Is the FX audible bc of the volume knob? Are 6sn7 tubes not as good as 5654 tubes? I am struggling to understand any logical reason for this.