The filter quality of the two parametric mid bands can be switched between two characteristics: Wide (Q 0.5) and narrow (Q 1.0), based on the proportional Q principle.
When designing a parametric filter, you have quite a range to choose from when it comes to the Q factor. After extensive listening, we decided to implement a choice of 0.5 or 1.0 in the xfilter 500, as these parameters always result in what this machine is focused on: Very musical processing.
You will notice that very narrow Q factors are not offered here. These days, most engineers we know of use digital filters for notching narrow ranges to get rid of problematic frequencies anyway. And this is for a good reason, as it works extremely well.