The All Metal mode is what I consider to be a zero discrimination mode. In a post I made back in June 06, I made this post, based on my comparing the different models of X-Terras and a very low ferrous target. Here is part of that post, as well as a link to the string.
Another post mentioned that the X-70 has more lower end adjustment than the X-50. Although this is true (as far as the number of notch segments used to cover the "iron range" for each model is concerned) the X-50 actually has the capability of "going lower" than the X-70. The X-50 can register targets with a reading of -9. Whereas the X-70 will register the lowest targets with a -8. And, as mentioned above, the X-30 "bottoms out" at -4. So, even though you can "fine tune" the X-70 to either accept or reject more defined notch segments, the X-50 should (theoretically) detect (-9) targets in a Pattern mode that neither the X-30 or the X-70 will detect.
http://www.findmall.com/read.php?55,356446,359352#msg-359352
And here is the link to a more comprehensive "follow up" post I made after testing various detectors with a piece of ferrite. Keep in mind, ferrite is magnetic, like steel. But it is not a conductor of electricity. http://www.findmall.com/read.php?55,362081,362081#msg-362081 and the follow up post http://www.findmall.com/read.php?55,362081,363809#msg-363809
HH Randy