If you aren't going to be listening at high volumes, there isn't a big reason to get the ULS-15 over two good subs of lesser output capability. BUT remember that you only really get the frequency response smoothing advantages of dual subs if you can place them where they will compliment each other's response. If you only have two predetermined locations for them and can not place them anywhere else, you may not get any benefit at all except for the extra headroom. The RSL subs look good, and I would also consider the VTF-1 mk3s. The VTF-1s have a tight CEA-2010 average of 110 dB at 40 Hz and above. That is very good performance in the music range. Although the ULS-15 mk2 is such a powerful sub it more than doubles that.
The thing is once you get something like a ULS-15 mk2, it is difficult to think of going back to small subs. I enjoyed the hell out of mine when I had it.