I have a sub with my AV/3. I'm not sure what to tell you. I think depending on the music it may be nice to have sub running. My reciever crosses the sub over at 80hz when it is turned on, I wish I had one that I could set to 60 because I think that would be perfect for these speakers. I usually just leave the sub on and let it handle below 80hz and the AV/3 above. But, I have some music I'll turn it off for, my Marantz has a 'SOURCE DIRECT' mode which bypasses the sub and sends full signal to the mains...I use that on some music. The AV/3 can play fairly low very well, but for the most part a real good sub can do the low stuff even better, that's what they are designed for.
Probably not a good answer to your question....kind of rambled.
Oh, I have built one of the SW12A sub kits for a friend....it was nice, but in my personal opinion the SVS I have has just as nice of a sound and capable of more output for the movie watching I do. If I were only listening to music I think I'd like to have a dual SW12 possibly, I think that would be very nice for music in a larger room. If you have a small room the single might be enough....again, this is for music, not movies.