JMHO, but I don't think adding a tone or modifying an existing tone would affect depth at all. Detection and ID are separate circuits from the audio portion. There were a few of us early owners who complained of excessive interference near powerlines, but I don't believe our opinions were given much weight. But if they did choose to address this issue, it's conceivable that overall gain may have been reduced in order to reduce interference falsing,
I have an "upgraded" T2. When purchased, it was r5, I had it upgraded to r6 while having a coil replaced that had a loose internal connection. While I mentioned electrical interference, I don't think my unit behaved any differently after the upgrade, it still falses in air in the same two areas that it did before the upgrade. Nor do I believe that I lost any depth of detection.
Who knows for certain, other than Tek, what was changed in the r5 to r6 versions. There's bound to be differences from one unit to the next, just considering variations in individual parts and components, which are often manufactured with tolerances of 10% or greater, as far as the actual resistance or capacitance or gain of a device might measure. So this alone is bound to inject some variability between one machine and the next, all other things being equal. Good final tuning procedures might mitigate some of this, but every tuning job might be a little variable also. To account for this, manufacturers usually specify performance at the lower end of this bell curve, but again, we don't know the actual tolerance for the T2's specs.
All I know is with my limited experience digging deep targets, I did not notice any immediately discernible loss of depth of detection or reduced sensitivity at shallower depths. Could some other variable be at fault? I've read the upgrade is mostly software, but also a few parts may have been changed. This may be where to look, as it injects the extra variability of parts tolerance and factory re-tuning accuracy.
Anybody else who's been through the upgrade notice anything??
-Ed