I have a VTA ST70 and a VTA SP9 TCLA preamp With a SP6 phono.
The ST70 is dead quiet with inputs shorted and when using an iPod or a PC as input. The hum starts when the preamp is connected to the preamp. I sent the preamp to Roy who could find nothing wrong with it and said it was quiet for him.
I have used cheaters to lift the ground from the preamp, switch the hot and neutral of both preamp & amp, no change. I even grounded the ST70 & lifted the ground from the SP9 which quieted the hum a little bit, but not much.
The hum is there with the ST70 plugged into the out jacks of the SP9 even when the SP9 is shut off, but the hum stops when the SP9 power cord is unplugged.
There is no hum when the ST70 is plugged into the tape out jacks of the SP9 with an iPod connected as an input.