So, I was playing at a low volume on channel 4 2xEL34 and 4x6L6 and it sounded like I lost half the power (lost volume and punch). Turned the amp off for a bit and now I'm not sure my ears are playing tricks on me and I imagined it. When I turned on amp it was making a barely audible hissing, white noise, kind of crackling sound. Any one know what could cause all this? Just bought amp last week.