SFL 2 humming issue

If any components (except the one you are grounding-amp. preferred) has a three-pronged plug on the AC power cord,this will set up a ground loop,
and therefore a hum, decause the is now grounded in two places-wall and interconnect. Add a grounding cheater to the plug to convert it from the three-pronged to the ungrounded two-pronged type.
Ground the tonearm grounding wire to the preamp chassis if you are using analog front end.

Go to the power supply unit and listen with your ear close. If it appears to be coming from the transformers, that is normal. I did hear that some of the transformers were louder than most. However, all of them are going to make some hum. There are two (dual mono) and they are the size of some small amplifiers.

You don't mention where the humming is coming from but I suspect the transformer. Have you tried to see if may be loose from the pc board. Also it may be a grounding problem. Do you use a pwr conditioner and if so what might be on the same line. Some gear may always have a little hum to it. As you mentioned you only hear it when all is quiet. My SonicFrontiers dac has a slight hum also but I have added a vpi brick over the xformer which seems to do the trick. This may be something you may want to look at. Best of luck and Happy NewYear.