I am with Bearti, although I cannot speak about the quality of the BOSS-NS2 noisegate ...
I have a Marshall JVM410 which is well known for highgain noise on the OD-channels on the yellow and red modes.
Also have connected the FX8 via 4CM and my OD-channels are quite as a mouse.
How do I do that: --> I use 2 Noisegates of the FX8 to dime the noise floor of pickups/ amp feedback AND noise from the pre amp section of the Marshall amp. Using BOTH noisegates may not necessarily apply to you. But the following description will show you proper positions of noisegates in a guitar signal chain ...
source 1 for noise/hum: pickups/feedback
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Set the Gate/Out in your preset as the FIRST noisegate. I have the following parameters. This may be different from your Marshall. Change by ear:
Treshhold: -75 dB
Ratio: 2,5
Attack: 1ms
Release: 100ms
Input Gate Out level: 0dB
Input Z: auto
This noisegate is I think the first FX in your signal chain.
source 2 for noise: the pre(amp) section of the amp
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Use the GateExp effect as your FIRST effect in the signal chain in the FX Loop of your amp (this is the signal which comes directly out of the pre amp). I have the following parameters. This may be different from your Marshall. Change by ear:
Treshhold: -30 dB
Ratio: 2,5
Attack: 1ms
Hold: 1ms
Release: 100ms
Low cut: 10 Hz
high cut: 20.000 Hz
Level / + Balance: 0 dB
There are other potential sources for noise/hum, e.g. cables, etc. which I have not adressed.
In addition, I like to point out the following: The more external effect you use, the more you cannibalize the smartness of the FX8-functionality and simplicity.
Any how: whether you use the FX8 possibilities to reduce noise or your BOSS NS-2, my comments should give you an idea where to place them within the signal chain
wolbai