Yeah, I've seen it peak at the -10dB setting and not at the +4dB setting, too. That makes sense… if the signal is at +1dB, it would be 11dB over the -10dB setting, but still under +4dB by 3dB.
Neo, let us know if you have any luck figuring out your noise problem, ok? So much of this depends not only on the pedal settings, but on the design of the loop in the amp. There are more than a few quality loops out there, but they're all somewhat different, not necessarily better or worse, just different. I'm sure that explains why I've found the Factors to be quieter in instrument level loops… it's the loop I'm using it in.