Low level hum from a cartridge (turntable) is common. Personally I am ok with barely audible at full gain, which as you say can become inaudible some distance away or when the volume is set at a realistic level. This will not harm your equipment, a ground loop will not harm your equipment. Hum can be caused by bad cables, sometimes different grounding methods, a ground loop. The most common cause of hum is an AC magnetic field (EMR) electro-magnetic radiation which can be present in the ambient air, or carry through the AC lines of your home. (and ground wires with a ground fault) EMR is generated mostly by transformers nearby, phone chargers, door bell transformer, thermostat transformer, your power amp, light dimmers, refrigerators, AC fans, a laptop computer power supply, a wireless printer, and even the step-down utility transformer on the power lines, or a street light, a traffic light control box. All of the above offenders can overwhelm audio gear, even when grounded properly and well shielded. That's also of great importance, shielding!!! Some audiophile cables are NOT well shielded, although I do not know off-hand if Blue Jeans is one of the bad ones. Edit: Blue Jeans looks ok, just visited their site, nice Taversoe connectors with spring "grippers". Some pre-amps are more subject to EMR than others. For instance, the phono input may be internally strapped to chassis of one brand, or a floating ground of another brand. Certain turntable manufacturers pick up ground through the Left channel RCA jack, others a separate ground wire to chassis. Under certain conditions, one brand turntable may hum with a particular pre-amp, but be perfectly quiet on another... sometimes two can be incompatible.