Swap the pre>amp ICs and listen for the hum to follow (or not). If it follows, then it's either the preamp or the source, and you then need to restore the pre>amp ICs and swap the source>pre ICs. If the hum follows the source>pre IC swap, then it's the source.
If you determine it's the amp, then as Joeylawn36111 says, you need to swap pairs of tubes L>R one pair at a time, restoring each swap if the hum does not follow, and repeating the process until the hum follows the swap.
If you determine it's the amp, then as Joeylawn36111 says, you need to swap pairs of tubes L>R one pair at a time, restoring each swap if the hum does not follow, and repeating the process until the hum follows the swap.

