IIRC, the threads on Max Flows & their tanks are non-standard (reversed or different pitch, whatever) so they'll only work with each other. I haven't owned a Max for about 3 years, so I may be wrong.
P8ntbllr99: You should contact WDP first to confirm that the standalone reg will work with the Crossfire tank. I've never had a Crossfire, so I can't give a definite reply re that make.