A hum problem can be very difficult to identify as there are so many factors to consider.
In my experience, as it relates to analog front ends only, I find the preponderance of hum problems are usually found in the (lack of or poor) shielding/grounding somewhere in the turntable, tonearm, and cartridge combination.
Another candidate would be tubed phono stages.
Again, these are the most likely based on what I've owned and serviced. There are certainly other hum inducing causes that are far too numerous to list.
In my experience, as it relates to analog front ends only, I find the preponderance of hum problems are usually found in the (lack of or poor) shielding/grounding somewhere in the turntable, tonearm, and cartridge combination.
Another candidate would be tubed phono stages.
Again, these are the most likely based on what I've owned and serviced. There are certainly other hum inducing causes that are far too numerous to list.