Not exactly sure if I am understanding what you are saying. Generally speaking, the ball valves on the Febco backflows are female threads on both sides, which would mean that it is threaded onto the actual backflow unit, and the outlet pipe is threaded into the other end. I am not sure why a threaded fitting would be soldered, that just doesn't sound right. By the L-joint I assume you are talking about the elbow just after the outlet ball valve. You probably will need to replace all that in order to get the ball valve tight enough, unless you are using a compression type fitting or a union. Bottom line, you shouldn't be soldering a threaded fitting in any situation, sounds to me like the ball valve it either not tight enough, or it is cross threaded or stripped. Might be an ideal time to have a plumber come out and take a look at it. Its good that you wanted to give it a shot yourself, but this type of work is truly fit for a licensed plumber.
Tony Posey
Ridge Run Landscapes