Interesting. It seems to be related to that first band somehow.

If I change that first band so that it's centered around 100-150 hz with a high Q (say, 20) and decrease it to below -27db, then the distortion goes away.

But if I leave that band as-is and make the SAME correction using one of the two unused bands (9 or 10), then it doesn't fix the distortion.

Not sure if that indicates a bug, because there's not a sudden moment when the distortion turns on or off. It's a gradual thing. And by messing with that frequency, I'm changing the response curve of the EQ overall...

At this point, one might be inclined to say that it's just the S/N of the unit, but with all bands flat, which is much much louder, there is no distortion at the same volume setting. Is there something I'm not understanding about how DSPs handle equalization? If I'm only cutting frequencies, then in theory, nothing should be getting louder, right, and therefore not distorting?