Excellent point. There are probably more coil compatible low freq Whites units (XLT, QXT, 6000, etc) on the market then any other detector models, so one would figure they would be prime candidates for the SEF coils to push depths deeper, separation left/right better, and also to handle mineralized ground more smoothly. If they'd do that I think it would breath new life into the old low frequency Whites line, as there are no DD coils for them, or larger coils than the 9.5" (there are a few examples but they are concentrics and don't get me started on how bad they are). The low frequency Whites penetrated my mineralized soil better than any other single frequency machine I've ever owned or used. Some of the best single frequency units on the market IMO, at least in my soil, but seems Whites has left that for higher frequencies and more complicated machines. Never will understand why they've done both these days. When they offer a low freq/simple computerized unit that still gets excellent depth, along with some bigger coil options, I'm going to be the first in line. They were my first love, and you never really get over that...
If Detech makes SEF coils for very obscure/less popular machines, like even the years ago defunct Musketeer that never seemed real popular, then how they could overlook the low freq Whites is beyond me. I'm suspecting there may be some issue with a DD coil on those units, because far as I know the only DD coil you can get for them is like a 4x6 trash coil or something.