Using an analog EQ on a stereo buss can be quite tricky, as only very few units offer the possibility to link their two mono channels for true stereo processing.
As a result, the user needs to fiddle around with many knobs in order to cope with the tedious task of matching the settings for both channels as good as possible. The problem gets only worse when the controllers do not have any steps for reference.
But while there are so many compressors in the market that can be linked for stereo operation, why are there almost no EQs which can do this, too? One of the main answers is component tolerances. Obviously, you want the two linked channels to behave exactly the same, but the natural tolerances that especially potentiometers and capacitors have counteract this ideal characteristic.
To make a long story short: The xfilter solves this problem by using computer-selected dual and quad layer pots as well as special low tolerance caps.