Hey Chip,
I've been using a pair of dbx 463x Overeasy Noise Gates with my MP-1 for a few decades now, and I've usually put them in the effects loops, just before the poweramp.
I guess it really depends on how noisy each piece of equipment is in your signal chain. For me, the most unwanted noise comes from the preamp itself with higher gain settings. When I use the ADA, Eventide, or Lexicon processors in the effects chain, they are very quiet units so, I can put the noise gates in front of the effects, and set the threshold to just trim the unwanted white noise out of it, and have my delays and reverb at the end of the chain so they are not affected by the noise gate.
You could try to experiment with the placement of your gate in various spots in your signal chain and see which works better for you. As for 60 cycle hum, at lower gain settings, it isn't much of a problem for me. I like more of an overdrive tone rather than a distortion tone, which helps keep the clarity of my guitars intact. When I do use a high gain setting for a blistering lead tone, I always step back to the overdrive tone right after the last note in the solo. That takes all the extra noise away right there and the gate, being already set up for that setting, does it's thing.
Experimentation is the best teacher
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I hope that helps