I've had my Spider Valve for a couple months now, and today it started to drop in volume randomly, like I had a volume swell pedal hooked up. At first it would fade out to a low volume and then come back up to where it's supposed to be at. Now it stays very low and randomly blasts at full volume. I have to have the master volume at about 11 o'clock to get any decent volume out of it. Then it randomly gets deafening and fades back down. Can anyone help me diagnose this problem? I don't want to have to buy new tubes if that's not the issue.