Rc2125

Not sure if this info helps any, but this is the direct info from the FCC compliance test/reports for the ML-105's and then Manticore:

Schoolofhardnox,,      had the same exact stuff going on with mine, at one certain huge beach area, regardless of literally every setting/mode/sens/gb attempted, just endless iron sounds and couldn't hit anything deeper than like 4-5 inches. Freshwater, Lake Mi. Then, at other beaches I went to,  did just fine and very deep.  The problem beach had multi layers of magnetic sand all throughout, but never had any issues in the past there, with hundreds and hundreds of hours with nox & then legend.  Due to an injury, haven't been able to go back and experiment yet, will be several weeks. 

Got a set of each, one with the Visation speakers and the first set he had with the piezos. The Speaker ones sound much nicer than any piezo does and are my wading headphones (speakers don't do so well when submerged). They are plenty loud, however, the lower mid tone is weaker than the mid and high tones,  actually too loud above 23 for me. My hearing is poor (military/shooting/motocross/etc/etc)  with tinnitus. 

The piezo ones look the same but are not as loud.  They are louder than GG or Koss' on a CTX, but still "lacking" for underwater use, imo. Found that i have to stop breathing (scuba) often to hear targets and have just been taking my Excal instead, ...bummer!  Had hoped Minelab would have surely made this detector have adequate volume capabilities (after the CTX was a Huge fail in this volume department underwater/waterproof).  

Will try the Koss yellows, not with high hopes,   if a set ever becomes available.......     

Have had several GG waterproofs and compared many times with Koss yellows in the past, so not high hopes for the GG's either

Wish there was a software "fix" to up the volume for shallow underwater use  ((  Minelab...  = HINT HINT ))