If you look under the hood of an audio equalizer, it is very likely that you will find a lesser or greater number of film capacitors. These caps use an insulating plastic film as the dielectric, which is drawn in a special process to an extremely thin thickness.
There are several technical reasons why film caps are a good choice for audio filter circuits, and generations of audio engineers will confirm that they sound great, too.
A typical representative of this component is the MKS film type, which is a great sounding cap to be found in many EQ designs. However, many MKS types come with a typical tolerance of 10%, which can have a negative influence on the channel accuracy of a stereo linked EQ.
For this reason, the xfilter 500 uses a special FKS type film capacitor which sounds just like a good cap should, coming with a maximum tolerance of 5% only. One more step towards a more accurate stereo image...