Need help with some crackling buzzing noise...

I need some help with my Ampeg J-20. Bought the amp new, played it stock, everything ok. Changed the tubes and made a new speaker cable, everything ok. Changed the stock speaker to a vintage 30, everything ok. Today I changed the speaker to a brand new jensen p12q and a newly microphonic v1 tube. Now when I turn the amp up, when it brakes up to a light overdrive when played hard ther was some weird kind of buzzing/chrackling/farting (it's hard to explain) coming after. It's just on some notes, a low g or a low b for example. Somebody has an idea what that could come from?
I checked the tubes, they are ok. checked the tube sockets and all the screws, everything ok. spekaer is screwed in right. Had a bit a Problem getting the cable shoe on the lug but they are on and the connection is good. Could it be the speaker that's not broken in? The connection on the speaker cable? (can't imagine) Or maybe because of the weber mass?? Never had that before.

New tube in V1 you say? I'd look there first. Try another tube and see what it sounds like. I bought an amp that is really particular about what is in V1 - if I use anything other than a EH 12AX7 I get an unpleasant crackly sound on a low overdrive setting - really bad at lower volumes, less cranked. Fortunately for me the EH brand tube sounds great.

I tried different tubes, JJ, EH, TAD, old Mullard, Tung Sol. Nothing changed. Maybe I should mention that it happens when the volume is at the break up spot tamed with a weber mass. Wasn't able to try it without the attenuator at home.

bump! I really need help with that!