So it appears you are saying that you are new to Xtol? Are you new to TMax as well? What other B&W developers have you successfully used with what other B&W films, if any?
We need photographic chemists to contribute but what it is about Xtol that makes it susceptible to the water quality that doesn't affect C41 and E6 films?
Have you tried printing? We still need to know more to get to the cause