The problem I see with the above posted specs, the SW-8MK2 drops out at 38Hz and that's more the type of sub one would have to help out weak speakers, not help out in the subwoofer department.
Personally, I don't think the SW-8MK2 will play and get along well with the RW-12d. See it in the light of a little league ball player trying to keep up with the high school players. The little league player may have all the heart in the world but their size just won't allow them to keep up with the high school players.
In my opinion, you'd be better served saving for and getting a second RW-12d and not using the SW-8MK2 as the SW-8MK2 is going mess with your hearing whenever the sound track drops below 38Hz.