I think that for music you're best off using just a very good pair of full-range speakers. You lose stereo separation if you use only one sub. By the time you've bought two high quality subs, you could have used that money to get some even better stereo speakers.
Even for movies, I question whether a sub is worth bothering with. If you like action movies with big explosions then it might be worth getting one. According to Denon, the LFE channel is hardly ever used in most movies.
You have to have error ranges quoted in specifications, or the measurements are worthless. As far as I know, speakers with true full-range performance have to be very big, with big woofers, simply because they have to move more air around. I know that is hardly a technical explanation, but you know what I'm getting at.