Here is a 4 –18 khz variable frequency VLF metal detector that may find a niche in the States: the Rutus Alter 71.
Ziggy Jinx Detector Testing (Facebook: Ziggy Jinx Metal Detecting) is currently testing the Rutus Alter 71 in the UK. MD-Hunter also has a write up (Rutus Alter 71 test. New 2016) which includes several non-translated videos. The Alter 71 was apparently released in the autumn of 2016 but apparently is not yet available in the US. I am not familiar with either the Rutus MD’s or of any US dealers. Non-verified & non-translated video(s) indicate the box may include both 11” DD and 9” concentric coils, control box and housing covers and a 6 cell AA battery pack similar to the Garrett AT series. Battery life is approximately 20 to 50 hours depending on the frequency settings; estimated price US $725 plus; the menu appears to allow you to toggle over to English. The battery door upper insert tabs may eventually break off with use.