Low level hum only with lp playback

Basic system:

VPI Prime turntable (cart is a Dynavector 20x2L) with
Purest Audio interconnects ( with integral ground wire connected to phono pre) to
Parasound JC3+ Phono Pre with
Cardas Clear Light Balanced Interconnects and separate ground wire connected to
BAT Preamp
REL R-r328 sub

I use an Eagle PSU with Roadrunner tach for speed control. 

So I'm getting a low level hum ONLY when the record is playing. You can hear the hum on the lead in groove - the sound gets louder as the volume goes up even if the music hasn't started.  However if the needle is lifted the hum goes away. 
I played around with the ground wires locations to no avail.
I've checked the leads to the cart for tightness and to make sure they are connected completely -they are. 
I've moved the speed control equipment to make sure they are no interfering. 
All my connections are tight.