I recently bought a Spider IV 75W amplifier from my local Sam Ash. I know it was on the sales floor, which means a lot of people had been playing it. I'm not sure what the reason for my problem is though. What happens is each time I turn the effects knob (located all the way to the left of the amp) to any effect, the volume spikes so loud that I think it's going to blow my amplifier. I then have to turn the channel volume all the way down and then back up to the desired level. I have to do this every single time I change effects. Anybody know why? Do I have a busted amp and need to exchange it?