Okay, I think the answer is #6: the NS-2 is not a noise gate. I guess I should have known from the name "noise suppressor", but there is a lot of conflicting information in reviews and forums. Even the manual says "The NS-2 works as a noise-gate unit, cutting the noise and hum that occur between song intervals or a break", which mislead me. After doing more reading, it seems like many noise suppressor pedals are not true gates, in the sense that they do not offer 100% attenuation. I have always assumed they do, so I learned something today. In that case, you were right. The combined hum of all the pedals plus the preamp lead channel is too loud, and becomes noticeable even though it is attenuated. (The hum is still better than when you bypass the NS-2, right?) Sorry if I mislead you with my own wrong assumptions! So your idea of adding a second noise suppressor makes sense. Or switching to a different pedal that offers control over the attenuation amount, or naturally attenuates more than the NS-2.