I have the RF25 and the RW12. I use mine for music and hometheater. The rw12 sounds great and does a great job for music. A little bit of port noise when you go down in the low 20's, however, that would meaningless if you do mainly music. The sub will keep up with them no problem. I am very happy with mine. Depending on what price you are getting the sub at, it will be a very good deal. Regardless, it is a good sub that will keep up with your rf15.
Other subs to look at would be the klipsch sub12, SVS PB10, SVS PB12, and the klipsch RSW10 which are all fairly close in price depending on where you buy. The RSW10 might be better for music, but i haven't had the chance to critically listen to it.
Hope that helps.
Good luck,
Jonathan