From the manual of the 5:25 Plus :
«The PRESET DEPTH control takes the place of the 5 EQ Sliders as the interface to the EQ Circuit when you’ve selected PRESET on the EQ STYLE switch. As you increase the PRESET DEPTH Control from 7:00 / Off (a flat setting equivalent to all 5 SLIDERS set on the Center Graph Line) through its range, the mids start to dip and both low and high end are slowly boosted - until at 5:30 (maxed out) - you have an extreme mid-dipped “V†setting. This dial-able enhancement allows for some subtle applications of the EQ circuit at the low end of the control that can be superior to
the SLIDERS for certain sounds because it acts as an EQ “mix†control at these lower settings.»