Need help with Ampeg J-20 crackling buzzing hissing noise issue/blocking distortion?

[mfcpj]

I have an Ampeg J-20 with a noise Problem. Its a weird crackling somehow hissing buzz when notes ring out. First I thought of the speaker or tubes or some parts rattle. Checked all the screws, changed speaker, changed tubes, nothing helped. Did mods reffered to hasserl to setup bias and removed a bit of the low end, still here. It can't be a volume issue as I tame the amp when driving it. More gain more noise. It's driving me crazy as I like to play my amps at the spot where they break up. After some research I came across blocking distortion, might that be the problem?
Here's a clip (with the focus on the noise, not the playing): YouTube - Ampeg J-20 Noise Problem/Blocking Distortion?

Here's a schematic: Prowess Amplifiers - Ampeg - Schematics - Ampeg J20 Jet Current Production - fender 6g3 brown deluxe Clone hand wired J 20 Jet

Here a picture: http://www.ampeg.com/products/diamon.../wiring_LG.jpg

Hope somebody could help me or knows a place where I can get help as I can fix it myself as soon as I know how.