I am one of the original field testers of the X-Terra 50 and I don't think that there is anything wrong with your machine. You might try a bench test with the coil out away from any metal, but frankly I don't put much stock in air tests in terms of determining the depth capability of a detector. What I use them for is to check target response, both audio & visual.
When I got my test unit I ran various targets over the coil (just a few inches) and made my notes as to the various responses. Then I went out in the field & actually detected to find out how the machine did in the field with regard to buried targets. And I was (and continue to be) impressed.
You will find several posts on this forum from users who are doing very well with the X-Terra's, and you can also refer to my post of 10/24/05 for additional information.
So I think that you will be fine with your new X-Terra. If you happen to have a test garden I think that would be a much better test for your unit, but if not I'd suggest just getting out there & putting the detector thorugh its paces.
If you encounter any other problems or concerns, please post your question(s) & I'll do my best to answer them.
Bill (S. CA)