First crimp your spades onto bare clean copper. Then if you wish, apply some solder to additionally seal the crimp connection from oxidizing, but don't solder onto the lugs' flat connecting surface where it meets the binding post.
Question for users of bare wire ends speaker cable
There have been discussions on the use of bare wire ends as being the best way to get a good quality speaker cable connection. The only downside is the need to re-do the connection often because of the corrosion of the copper. As a part way measure can the ends be soldered or something else, to cut down or stop the corrosion without adversely affecting the signal??