Hey Daniel,
The Iron Disc was a bit weird with the extreme. Have you fiddled with the Level Adjust? Excerpt from manual below. On the GPX the Disc gets its own expanded control and everyone I know thinks the GPX disc can actually be made to work halfway well. Never use it myself.
The Commander coils are heavy but the best you can get as far as how they work, in my opinion. Have to say I am doubtful this will solve your issue as the GP coil is basically a Commander coil also. Hate to tell you the GP Extreme was kind of like Fishers these days o there were hot one and cold ones. The GPX series seems very, very consistent by comparison.
"Level Adjust Control
Level Adjust gives the user the ability to select the sensitivity and
likelihood of a target being discriminated as ferrous when used in
the Disc mode.
This may also affect how the detector responds to ground
mineralisation compared to metal targets when used in Disc. Mode.
There is usually some overlap between signals from useful targets
and some types of iron rubbish. The user may set Level Adjust to
select the amount of discrimination between two extremes: • (Fully anti-clockwise). All targets detected, but some iron will
give a normal signal (as per Figure 5.4).
• (Fully clockwise) Little iron detected, but some small nonferrous
targets potentially mistaken as ferrous.
Turning the control anti-clockwise increases the strength of a signal
required for iron discrimination to be activated.
The Level Adjust also affects the Tracking mode.
When set fully clockwise, it is unlikely the detector will 'balance out'
genuine targets, but some ground noise may still give false signals.
When set fully anti-clockwise, the detector will 'balance out' ground
noise effectively, but some faint, weak target signals may be missed
after repeated sweeps of the coil.
HINT It is usually best to dig out all targets and only use the
discrimination function in areas where a large amount of rubbish
makes the digging of everything impractical.
NOTE Discrimination functions will only work with strong,
positive target responses. Weak responses will give normal 'all metal' type signals.