Neil, Thanks for your post. I agree with most of what you stated. The only thing I'm not convinced of is that on-going question of which is deeper, Prospecting or Coin/Treasure? I'll share my thoughts on that matter at the end of this post.
In an effort to answer your question about comparing all-metal with zero discrimination......Consider that the term ferrous represents a substance containing iron. Conductive means the substance's ability to transmit electrical current. To me, All-Metal represents a detecting mode where every target with either ferrous or conductive properties will be detected. No matter how ferrous or conductive they are, it will detect them. I say no matter how ferrous because highly mineralized ground can contain varying degrees of ferrous properties. (iron) Adjusting the GB will further "cancel out" the effects of these ferrous properties caused by mineralization. With zero discrimination, the detector will only respond to targets that register (ferrous and conductivity) levels within the parameters set up by the designer. That ain't a bad thing as it should encompass virtually every (non-soil) target we will encounter. For those properties that are not included, we'll never hear them to know what they might have been!
My opinion on Threshold.....The threshold tone of the X-Terra is a separate tone whose level of sound can be adjusted to the user's preference. This tone allows the user to hear a rejected target "blank out" while hunting in a discrimination Pattern. For those hunting in All-Metal mode, it provides a background tone to let us know that the detector is still working! It ain't magic. And it is not a replication of what the "coil is actually "seeing". In fact, it can't be a replication of what the coil is seeing. If it were, and there was nothing under the coil, it would go silent. And that really wouldn't be a threshold, would it? Back to whether or not it is "fake". From the point of view that it is a synthesized tone, I suppose it might be considered a "fake" tone. But if we hold that to be true, aren't all the tones produced by the detector fake? If they weren't, how could the user set the number of tones they want to hear? One tone, two tone, three tone, four tone or multiple-tone? A nickel will read a 10 or 12 on the TID. But with the X-70 set at two tones, it will sound a lot different than if you were set to hear one tone, three tones, four tones or multi-tones! That nickel is still the same old nickel, no matter how many tones you are listening to. If the tones weren't "fake" (or at least synthesized), how could they be changed for specific notch segments?
At the beginning of this post, I said I agreed with much of what you said. The only difference of opinion we might have is on the depth of detection between Prospecting and C/T. For the type of hunting I do, I am not convinced that the Prospecting mode operates deeper than the C/T mode, using the same sized coil. That don't mean you are not correct. I just haven't been convinced of it! I know my thoughts might be biased since I don't use the Prospecting mode for actual coin or relic hunting and only use it for target sizing and pinpointing AFTER I hit a target in C/T. Regardless, IF the Prospecting mode does hunt deeper, (and it might?) I guess I consider the "trade offs" for a coinshooter to be well worth using C/T. Variable defined tones, additional TID information etc. You gotta love the flexibility of the X-Terra!!! JMHO
HH Randy
proved it to me. I do wonder what the X70 in Prospecting Mode would do in the VA areas of bad soil on relics. I also wonder if some users are put off by the term "Prospecting Mode", not realizing what it could provide in certain circumstances. 
