“... Now the buzz is louder and intermittently it gets louder.”
A constant buzz could be ground loop related unless you can actually hear a buzz when you put your ear next to the amplifier transformer. A variably intermittent buzz is more like a bad transformer or poor ground connection somewhere in the Audio chain. Try disconnecting the preamp and run the amplifier by itself and see that makes a difference in the loudness of the buzz.
A constant buzz could be ground loop related unless you can actually hear a buzz when you put your ear next to the amplifier transformer. A variably intermittent buzz is more like a bad transformer or poor ground connection somewhere in the Audio chain. Try disconnecting the preamp and run the amplifier by itself and see that makes a difference in the loudness of the buzz.