I just got a used Iamp 500 . When I turn the main volume up to the 9-10 range I get a lot of white noise coming through my cab regardless of whether the tweeter is turned on or off. It is different from hiss. It's white noise and I can hear through the speaker. I can also hear noise and hiss when using headphones with the amp regardless the volume level. Is this normal? I dont remeber having that much noise on my other amps.