Adding a Second Sub

I am debating buying a second sub to have stereo subs. I have Sonus Faber Extremas and a Rel Stentor. I would really like to hear from people that have done this and what they thought of it. Were there any drawbacks? What were the advantages? Did it make a huge difference?
You have a great sub, so two Stentors will be even better. Should make the stereo image even better. There are no drawbacks I know of other than cost and whether the space can accomodate two subs.

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.

Haven't experimented with multiple subs myself, but an interesting article you may want to copy for insight was written by Tom Nousaine and appeared in the June 96 issue of Audio magazine. Don't scoff at the date. The article is an excellent discussion of room nodes behavior and is well grounded in not trying to beat the laws of physics. He titled it "Two subs in a corner beat five in the round".
Re: directionality of subs:

I have a REL Strata III in the right corner of my listening room. On the 4th track of James Taylor's Hourglass CD, beginning at around 3:50, there is a drum roll which goes from right to left on the soundstage, with a big whack when it reaches all the way to the left side. You'd never know the sub was on the right side. Absolutely amazing. Try it yourself.

Sugarbrie, I listen to a lot of classical music and am glad to hear that placement of my sub in the right corner is correct!
Deffinately two subs. I added 2 B&W 800 Matrix subs to my
801 III and will not go back. It was a match problem for
the 801's and the only seemless blend was another Matrix
12" sub. I have them reasonably low, but can really hear
and feel the difference. If it plays well together, go for
the lows.
If you are crossing over low enough, then you don't need two subs for stereo as Faberryman and others have pointed out. But two subs often gives a better result - the main reason being that you can get smoother response. Some subs use two drivers, one on each side, and their separation by a foot or so helps to get a smoother response. It is just that each driver will create room nodes and anti-nodes in different spots and (hopefully) cancel each other out. The downside is that set-up requires even more patience and time than with one sub. From experience you will very likely get a significant improvement with two subs, but two Stentors is a big investment in the bottom octave.