Joe,
First let me tell you I don't do business with Hi-Tech but have friends that do. Hi-Tech has a very good reputation and that comes form people to do know what they are doing. So, I have not ax to grind with Hi-Tech and only have nice things to say about them. I have a Jewelry Store and manufacture Platinum and Palladium Jewelry and buy/sell lots of gold every year. I Just have had my fourth XRF machine installed and have been upgrading for several years as better machines come into the market and have sold all my previous machines to friends. I also have two Specific Gravity scales that are accurate when you are dealing with solid masses and have two induction melters so toys, I have. I cash out several times a week in $12,000 $17,000 area and usually agree within $20. on my cash outs with a different Refiner.
You got several potential problem areas. First, most XRF machines are set up to read a tin/gold alloy. Lower priced XRF machines are matched to known standards which Tin is usually not in a common gold standard. The XRF is going to match it to the closest known standard that makes up a 100% of the metal so you start throwing uncommon mixtures at the machine, it is going to be off and sometimes wayoff. I have been experimenting with Rhodium and my former machine did not have a Rhodium standard in it, so it was WAY wrong. Second: Tin is very light like Zinc and I am sure it moves around a lot. When I melt 600 grams of gold in one of my Induction Melters, in just a few seconds, gold stratifies meaning it goes into layers. If you pour the metal into an ingot, you are better tilting the ingot and get a wedge. With a wedge, you measure all 5 sides and get an average. Third: If you run a S.G. test on the gold, you are going to have an seperate test that will agree closely with your XRF test to see if you got a good reading.
Now, with all that said, I am willing to bet that Hi Tech is correct and you are wrong. I have no axe to grind and if you want me to run the metal for you, I am happy to give you another opinion on what it is. I understand your frustration and know that you are just trying to get the most for your labor. I have studied Pin Samples and methods used to determine the fineness of a gold bar. I understand how XRF's work and how you can fool them. I have cut many of gold wedges and Ingot's in half to run the inside and to see what gold does when poured into bars. All I can tell you is you can get a lot of help with GRF guys. I know I have been in your shoes and keep learning until I found the right answer for me.
Good Luck and we all have been there!
Dan