As an XS reg owner, I feel your pain. However, pretty much any dive shop can service their regs. The problem is, if they need to replace parts, they have to order directly from XS and it takes like forever.
While they can, it doesn't mean they will. If they don't have the service manual or have not been trained or authorized by xs in writing, they may not want to take on the liability.
I could service all of their regs seeing the design and being able to find the manual for the one. If I wasn't retired from doing service work. But, I'd have to go through another dealer to find parts. Then there's the issue of would xs back me if that reg developed a problem. The answer to that is no. I was not a dealer for them. Getting parts from a dealer would not be a problem if they had them.
But I already found an issue with one spec in the manual I was able to find.
And then there's the whole will the shop service regs not bought from them?
Many won't. They get all pissy about it.
I didn't care where people bought their regs that I could service and was authorized to. It was incoming revenue.
Then there's the issue I saw more times than I cared to of shops saying they could service a reg and screwing it up because they were not trained on it, didn't have the manual and specs, and even put in parts from other brands that failed prematurely because they weren't right.
Just because they should be able to service it, it doesn't mean you should let them.