The PCBs of the xfilter 500 have a total of four layers: Two of them are used for the audio circuitry, one is for power supplying traces, and the last one is a dedicated ground shield layer. But what's the idea behind this?
The filter networks of the individual bands of the EQ become higher in resistance the lower their specific frequency is set. This makes them more vulnerable towards hum which is caused by electric fields.
As the additional ground layer is placed extremely close beneath the traces which carry audio, it is very efficient as a shield against unwanted noise interferences.