If the threshold is nulling out a lot the first thing to do is try slowing your sweep way down. If that doesn't help then lower the sensitivity. Certain ground minerals I've ran into require an ULTRA slow sweep speed to keep it from nulling. That's a good way to avoid going too low with sensitivity. Some places I hit will want sensitivity all the way down almost to the lowest setting (say past 3:30PM on the dial). Often you can avoid going that low by going even slower with the coil. If I have to adjust sensitivity that low I'll usually just throw it into Auto because Auto will get as much depth as that low of a setting yet provide a much more stabile threshold.
On the volume thing...One thing I noticed is that if I crank volume as high as it will go I can't tell how deep the target is anymore, mainly because I don't have any practice at it. Now is it a trashy shallow target that's causing it to act funny or a deep coin that's the reason for the unstable signal? Using these Sony headphones I have yet to hit a deep target that even low volume won't let me hear just fine, but that might be because of these headphones. I almost never run full volume as it's just too loud without a limiter circuit for these headphones. When I get those Sun Ray golds I'll run full volume, but the Sonys have the best audio I've ever heard on a machine. Hoping the Sun Rays are just as good.
One thing I've noticed is that on a real deep coin as you wiggle or short sweep over it the GT seems to "amp" up the signal. I'll hear a type of white noise in the background like an amplifier has been turned on. Stop wiggling over the target and I can hear the audio return to normal. Anybody else notice this? It's not static or such, just you can tell by like a high pitched trebble quality to the audio.
