I think it could be an earth loop problem.
Probably related to the SL1200 power supply, linking in to the arm, feeding the phono stage, and in turn linking to the preamp.
I returned the iPurifier, and have a Gnd Defender instead. So the power amp, pre amp, phono stage, DDC and TT PSU are plugged in to my mains block (Furutech TP60e). The Gnd Defender is plugged in to my phono stage. The hum has decreased dramatically.
I have the volume turned up way beyond my normal levels, and I can hear it now and again, so is it a ground loop?
Really not sure. My next step may be a different (filtered) mains block, like the Puritan Audio jobbies, as this is driving me mad.
I may swap out my cables from the phono stage to pre for some coax based cables, in case it is the shield (connected at one end) acting as an antenna.