I have used Braze 450, a type of silver solder, just with the standard flux and have silver soldered stainless okay.
I must say, however, I think you are going in the wrong direction. It is very difficult to do a nice clean joint with silver solder on tube especially using a gas torch for heat. What happens is the tube gets hot enough around the joint that the silver solder flows out away from the joint and you are left with a film or lumps of silver solder on the tube adjacent to the joint.