Did I talk about running the knobs in the center position? I don't recall that, but then again, there's a lot of things I have trouble remembering anymore. :wink:
The 4220 is a cut-only unit, like all passive EQs, that appears to be designed for tuning out "humps" in room response. It's intended to be run at the zero loss position except for those frequencies that need to be "tuned out." It's highly unlikely that the curves of each band have a perfect enough overlap to give a truly "flat" response with all controls at center. More probable is that centering all controls will give you some low amplitude peaks and valleys up and down the audio range. There are several passive EQs that are designed to be flat (relative to their insertion loss) at the center position, but this is not one of them. The passive EQ design I published here a while back, as well as others such as the Cinema Engineering 4031B, Langevin EQ251A, etc., are capable of a flat response with boost and cut possible, relative to the nominal insertion loss.
An EQ like the 4220 can be used successfully for music, but it requires a different way of thinking. You have to think in terms of what needs to be attenuated, rather than what you want to hear more of. As an example, if a male vocal track sounded "boxy", you would apply a cut around, say, 300 or 400Hz, rather than attempting to boost the 5kHz "presence" range. Actually, you'll often get better results by thinking subtractively regardless of the particular type of EQ you're using.