The strange thing is that I can hear the crackle when ALL the controls are turned down! Of course it gets louder when the controls are turned up!
Well this suggests that it's in the power amp section or the power supply, and getting louder when the volume is turned up suggest it may be in the power supply.
I had no trouble finding a circuit (attached) but I can only find a copper-side pic of the board, however it looks to me like the main power supply filter caps are mounted on this PCB. A fairly common fault due to the large mass of these components is fracturing the PCB trace and/or solder joint where they are soldered to the board.
It is almost certainly going to be a fractured track or solder joint, and highly unlikely to be a transistor itself - this is not how they commonly fail.
says, it's a patience job requiring more of the same. I've found that a single chopstick tapped on the board gives about the right level of impact to isolate these sorts of troubles.
Good luck, and let's know what you find.
just beat me to it