A yes to Step 3 means that you very likely have a ground hum.
“If steps 1 and 2 were “no”, but removing all inputs eliminates the hum, it is likely you have what is called a ground loop.”
Although it is not recommended you can try lifting the ground pin to one of your items like the Preamplifier as a test.
If the hum completely stops then clearly there is a difference in ground potential between your two items.
The tutorial recommends trying the Humex ground line filter:
If you continue to have this issue then please reach out to us directly at firstname.lastname@example.org and we’ll be happy to assist directly.
Thanks in advance,