Check my long winded thread on squeal issues. [MODERATOR – that thread should be moved to 2 strok from Effects…]
You have what sounds a bit similar to what I had. Maybe this helps, maybe not, but note that the weber OT you’re using has the blue and brown primary leads swapped VS what the Dave Hunter spec’d hammond has – so if you put blue and brown where DH says with that Weber, you’re going to get squealing at some combinations of higher knob settings.
Took me forever to figure that one out, since I was checking my wiring and trying all sorts of other things.