I don't think it will help to turn down the threshold.
In the disc mode turn sense all the way, or almost,
and swing your coil slowly and strait toward and
away from the object. I can get pretty close with
my V. The Compadre does better, with less power.
If that makes any sense.
In all metal mode, go slowly toward and away. The
machine will detune itself as you go toward the object.
Listen closely and there may be a little wiggle in the
audio if another conductive object is present.
I have given up on using disc for this chore because
that cuts out lower conductive targets.
It's like masking. You can use disc and get some
high conductive targets, or you can listen very closely
and discern the little noises that the V is making.
For me it is like attempting something that seems
impossible, but somehow it works.
Or maybe it's just really getting to know the machine.
But I am fortunate enough to have a Compadre. That
is usually what I usually assign to going around metal
objects and reinforced concrete.
Sometimes, less is more.
HH,