When I test new machines for Kellyco, I'm not really concerned about depth of one machine compared to another since there are far too many external variables that go into determining "how deep" any machine can detect a target i.e. soil composition, target size and metallic composition, orientation of the target in the ground, coil size and frequency of the machine as a few examples. A detector that can find a target at 12 inches under one set of conditions may not be able to see that same target at 6 inches under a different set of conditions. Therefore to report that a detector can find targets at a given depth is very misleading.
I'm more concerned with and impressed by durability, reliability, good functional controls, ease of use, sensitivity and separation. Sensitivity being the ability to detect those smaller targets and separation being the detectors ability to separate and pick up targets among junk. Both of which are partially determined by the coil and the machines recovery speed as well as the processor's software design.
Others may have different criteria by which they judge any given detector but as I always say, this is just the view from my foxhole.
Soldier on...