I don’t hear any loudness change myself on the DB4, but the metering definitely changes when the tone controls on that mixer are changed from filters or EQ to iso mode and it has no bypass mode when all three of the tone knobs are centered. Isos do 100% produce a fixed amount of phase distortion when they’re in the path and this can be seen on test tones run through all of them. That causes a little summation, a little cancelation, some other stuff. If that’s not that you’re seeing, then it might just be a slight difference in the way repeat levels meter over time on the x1800 meters. Might just be that the meter doesn’t quite register the first peak as accurately as the later ones. Is it really that problematic? I mean, if you’re right on the margin of getting into a metered level you don’t wanna be, like into the safety headroom, then I guess you can back it off a touch.