Emorrisiv, sounds like you have a grounding problem. Make sure the arm ground, transformer ground and cable shields are all good. You can tie the arm ground to the SUT ground at the SUT's input and that should be consistent with the ground at the output of the SUT.
There could be a loading issue, but I would not mess with it until you have the hum problem fixed! If the ground is not right there could be ringing in the cable (anyone how has worked with oscilloscope probes knows what I am talking about); IOW the hum and the sibilance could be related.
There could be a loading issue, but I would not mess with it until you have the hum problem fixed! If the ground is not right there could be ringing in the cable (anyone how has worked with oscilloscope probes knows what I am talking about); IOW the hum and the sibilance could be related.