Well here's a riddle for you then...
Pulled the speakers out and did the push test you described for me. The result? No noise. No rubbing or clicking. No NOTHING.
I thought it was a bad connection so I tightened everything up and checked everything for looseness. After everything was fastened down, I plugged in the amp again and after a few moments, the crackling and whoompfing started again. BOTH speakers are doing this.
I did notice that this ONLY happens with the dirty channel, and when I play loudly and hit chords hard. I recognize it immediately because the sustain gets killed right away.
Anyways I turned the amp off right away and I proceeded to practice for the rest of the day through a Marshall 1960A cab I borrowed from a friend. The amp sounds great and performs flawlessly. No whoompfing or farting to be heard.
So the push test produces no results, it's not a bad connection, it's not the amp. So the speakers act as if they're blown (on the dirty channel only) but do not click or rub when pushed. Could they be blown and not seem like it from the push test?
This is so damned frustrating. I have a theory as to what is causing this but I'll reserve it until I get an answer from J.
Your move, J.