I haven't tried the MXR Smart Gate but I have used MXR Noise Gate/Line Drivers which aren't great.
I have a Boss NS-2 and it does tone suck (as with most Boss pedals)
I tried The ISP Noise Decimator and hooked it up after my pedals going into the Amp, it wasn't that great but it didn't tone suck, so I exchanged it for the ISP G-String Noise Decimator.
Your guitar plugs directly into ISP Guitar In, Guitar Out goes to your pedals before the Amp, your Amps serial FX Loop Send goes to Dec In and Dec Out goes to your FX in your loop, your final FX in the FX Loop goes back to your FX Loop Return.
With this connection the ISP G-String senses the string decay and does and excellent job of controlling noise. I however still use the NS-2 at the end of my FX chain hitting the front of my Amp followed by a CAE Boost/Line driver to revive some tone loss. I've always used a noise gate in the front and another in the FX loop to keep noise controlled.
I may replace the NS-2 with the ISP Noise Decimator just for an improved tone.
My guitar setup is JVM 410C and custom pedal board and no racks hence why I didn't go the ISP rack solution.
I do use racks in my home studio which is an extension of my guitar setup but not taken to gigs.
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