My hartke 3500-head make quite some noise. But when I turn turn the tweeter of my peavey TX410 down, the noise is gone...and when I turn the tube control of the amp down, the noise is also gone... I'm pretty shure that the noise is in the amp because you can hear it when using the DI-output. The tube isn't the stock one. I replaced it 16 months ago... What could it be? The tube itself, or some dust in the tube socket? Can I fix it myself? Thanks!