You could use the seismic with the 13ultra, but it would be very tough to place and tune them to sound right, certainly above my experience level. Even with proper tuning, the 13ultra is so good I don't know how much you would really get out of it. You would probably be best to just sell the seismic12 and use the one 13ultra. If you do decide to sell let me know, I have a friend looking for one. He lives in Mtl as well so it would be a local pickup. PM me if your serious.