This is an interesting question and 20 people will answer it 20 different ways.
I am really missing the Z as I felt very in-tune with it and had great confidence that I was not missing anything. Blind and naive confidence of course ? but a lot of this game is having confidence in your gear.
I found that anywhere up past a Sensitivity of 12 or 13 would get noisy with my unit but EMI was the factor more so than ground noise. Even with the coil dead still it would be pinging and bopping and zinging - every now and then I would even get a weeeoooWW. Somedays were better than others, stormy days were horrid, over head air traffic created short term issues.
I assume you have an SPO1 and I had a B&Z but brand is probably neither here nor there with this. I used to run my Volume at 1 and then turn the booster right up to my desired volume. JP once wrote that he felt turning the Volume up by 1 was akin to turning the Sensitivity up by 2 when it came to increasing the unwanted noise (JP please correct me if I have taken that out of context). These settings allowed me to up the Sensitivity just a little more to try and brighten those faint signals - but not so much that there is too much noise to concentrate or hear through. I didn't run a dead smooth Threshold but it certainly erred on the smoother side of things.
My noisy detector analogy is a tumble dryer with a 50 cent piece in it.
Just a 50 cent piece - you will hear easily even if you are 2 rooms away and have a low Sensitivity to that specific target.
A 50 cent piece and ten 5 cent pieces. A lot more noise to filter out, probably hear the 50 cent piece most of the time and not too fatiguing to listen too. Probably make out the sound of the 50 cent piece 1 room away.
A 50 cent piece and one hundred 5 cent pieces. Bombarded with noise. Very fatiguing. Stand right next to the dryer and you will probably hear the 50 cent sometimes but there is so much else going on you start second guessing whether you actually heard it or was it just the rest of the background clatter? And then you start going faster, turning down the volume because it is so draining and then you end up at the pub ?
The only time I ever used a high Sensitivity that was obnoxious was when I was sure I had a tiny target and I was having trouble locating it. Going from 10 up 17 or whatever really livened up the signal to help pin point it. Or where I had a really small area that was an old timers wash up area and it had tiny, tiny pieces that the high sensitive also helped with. But it was not a sustainable way to detect (for me).
My 2 cents worth. Maybe something in there will help. And then how it all relates to the X coils I am not sure.
I hope the X coils work out for you. Pat's patch lead certainly looks the goods ?