I have an Ampeg SVT-2 Pro from about 2000 or 2001 and the midrange selector control (five position knob) does not appear to be working. I also have an SVT-3 Pro which essentially has the same preamp and the selector on that makes a pretty big difference. On the SVT-2 Pro I hear no difference in the sound between selections whatsoever. I'm wondering if it's a defective switch or could I be lucky and it's just a little dirty? I don't have the schematics but even if I did I'm not smart enough to read them I'm going to carefully take it apart and check things out but any tips you might have are appreciated. Thanks.