Welcome to the Forum.
Congrats on your M3! Another rescued from the landfill.
Crackly vibrato? Swap the 6AU6 tubes positions. They have covers and are next to each other. The tube # is stamped into the metal right in front of the tube.One is vibrato,the other non vibrato.
If the problem becomes good vibrato and bad non vibrato it's a tube/amp/switch issue.
Upper manual faint? Volume soft switch shorted maybe? Vibrato upper switch dirty maybe? Ground wire from generator loose? Bad connection or dendrites on the yellow and green wires in the matching transformer?
Have you downloaded the service manual? It's free at Captain Foldback.
I'm sure to have missed something so others will be along to help out too!