Hi --
The DGX-670 has two levels of EQ: per-part and master.
Per-part EQ is a so-called shelving EQ which rolls off (negative gain) or boosts (positive gain) low frequencies and high frequencies. If you're doing a mix or playing against styles/MIDI, I would recommend starting with per-part EQ first.
Master EQ has five frequency bands and is partly parametric (selective). However, Master EQ affects all of the components in the sound: keyboard parts, styles, MIDI parts, etc. It is a system-level effect.
Even though you can set one of Master EQ bands to 55Hz specifically (with a negative gain to attenuate at that frequency), Master EQ will affect style parts, too.
I noticed that Master EQ has a "Light" preset. Maybe try that? I play Genos and almost always use "Flat".
Just some thoughts -- pj
DGX-670 Reference Manual, page 71
The block diagram on page 74 might be helpful, too.