I don't have the experience on the other Tesoros to say much more about it. The Golden is not a deep machine. Occasionally, under great conditions, I have found deeper objects, mostly old pull tabs, as deep,as seven inches, and sometimes I just stopped digging because I was in a park and did not want to dig any deeper. The point I was trying to make is that the nail board test reveals a great capability. I think most Tesoros from what I have seen and read would have that great separation capability down to the limit of its depth capability.
You do have to agree that it raises a bit of doubt about an expensive machine when it cannot separate well on the DBT. It tells me that when that kind of machine finds a deep target, it probably was not being masked by other junk items.
To be fair... I recently saw a YouTube vid of the new Tesoro 11x8 coil performing pretty badly on the nail board test. I would upload it, but do not know how. If you simply search "Tesoro 11 x 8 nail board test" a vid will come up by "Texas Tiger Digs" using the new black Vaquero and the 11 x 8 coil. In my opinion, I thought the new combo failed the test. That being said, he is doing the test in doors and I don't know what is in the floor under the test board, that might explain it, but maybe not.