Here is my take...and experience.
Tesoro has stated many times that any unit with manual GB like my Vaq doesn't need to be calibrated or "tuned".
Don't know for sure but I assume this tuning process is actually adjusting the GB, sensitivity and disc range internally to factory specs which would be neutral settings for general areas of the country.
The ones that don't have manual GB can be sent in for this process but under most circumstances it isn't really necessary because many have switched coils and not have seen any problems or noticeable lack of depth or sensitivity.
I haven't.
People being as they are it drives some crazy hunting with a tool that is not calibrated to optimum standards and settings, even if they are using a well working detector in the back of their mind the what if's will drive them nuts.
"Maybe I can get another half inch or more if I send it in".
I have read where a few really anal owners sent in a bucket of soil with their unit and coils which sounds logical to them if they have some hot dirt and I can understand that.
I have mineralized soil plus an extremely unusual amount of iron in my local dirt and I have wondered what they can do in my case, but I still seem to get to a decent depth where good things are hiding with my Vaq, my Compadre too, so I can't see spending the shipping money for just that unless I knew for sure it would make a major difference.
Of course there have been instances when for whatever reason detectors have gotten way out of whack and this service will definitely bring them back into line, but look at the many more owners that have Silvers, Cibolas, even adapted Compadres to other coils and report back with perfectly good, well working combinations no matter what coil they mount.
A few owners get inside and do their own adjustments and it works out.
Others screw them up totally and the only way they can get them fixed is to send them in so it is nice this service is available whether you have to pay for it or not.
When Tesoro had their blanket, "we will fix or tune up any unit we ever made for free no matter who owns it policy" how many bought used units and took advantage of this service and sent them in just for the heck of it...it was tons.
They finally did the math and changed this outdated policy and will still do it for original owners but second, third and fourth hand units will cost you at least $50 for them to spend the time to check it out.
That slowed down and reduced the the amount of detectors sent in I bet, I also bet most of them were working just fine and dandy before they were sent in.
The consensus is switch coils if you want, Tesoro coils would probably have the least chance of issues, and only consider sending them in if you encounter problems.
This is what most do and seem pretty happy from what I have read and noticed.
