Well, this is a touchy question because everyone does these things differently. I've found these to be MOSTLY threaded in. Do you have an access panel to get to the tub plumbing? If not, maybe you need to make one. I've owned a number of houses where I had to make an access. Anyway, if you access the plumbing, you may be able to see if it IS soldered. If it appears not to be, have a helper grab onto the fitting back there that the nipple is attached to while you attempt to unscrew it. With joint putty on it and it being in there so many years it will be REAL tight and appear to be soldered.