I bought a used 73 SF Dual Showman Reverb at a great price. It sounds great. However I am trying to figure out the master volume and also to see if there are any previous mods to it. On my amp with the master volume turned to zero I still get sound and plenty of it. Is this normal or should it be muted with the MV on zero? As I turn up the master volume it seems to sound more open and the highs seem to come to life. I am clueless on Fender SF amps as I have never owned one until now but to me the master volume on my amp almost acts like a presence or tone control. What am I supposed to hear on a stock SF amp with a Master Volume and while using the master volume what tonal and volume changes should I hear IF everything is stock and with no mods? Thanks.