For those others reading, one good sub is better than two cheap ones; so if you want two subs, but cannot afford two good subs for now, do like Perfectimage and get one for now and save up for the second. With one sub for classical listeners, I recommend you put it in the right corner, becaues the bass section in most orchestras is on the right side of the stereo image.
I find my REL sub does the opposite of Dr_Joe, more dynamic and better separation with the sub. All instruments have a sonic signature larger than the main sound. The better separation happens with the sub because the sub fills in the lower part of the frequency signature of each instrument that the speakers don't reproduce well. The sub must be integrated properly for this to work. A properly integrated sub will be invisible. You won't be able to tell where the speakers stop and the sub takes over. Integrating a sub takes time and lots of listening, but its worth the effort IMHO.