I just looked up the sub12. Nice sub. I don't know about pairing those two together. Maybe someone with more experience could help you.
If you feel your seismic12 is lacking, why not sell it and get an SVS PB13 Ultra? That thing should comfortably outperform the sub12. (Im basing this assumption on the fact that the 13 ultra outperforms the Servo15)
If you actually do consider selling your seismic12 let me know, I have a friend looking for a used one.