oldschool1,
Is this what the inside of the amp looks like?
http://www.studio-22.com/anthem/images/Anthem%20MCA%2050%20-%20Inside1.jpg
If so notice the two toroidal transformers on each side. Pull the top off the amp and check the center mounting bolt and nut for tightness. Make sure the round top plate is seated properly on the top of the transformers. Maybe one of them is loose.
After checking them, to make sure they are secure, power up the amp and listen to the two transformers. If the hum/vibration is still there listen to each transformer individually the best you can. First verify the hum is coming from one or both of the transformers. Is one louder than the other? If so recheck the mounting bolt and nut. Make sure the round top plate is seated properly on top of the transformer.