I recently purchased a Stearns & Foster Camdenhurst Pillow Top mattress.  The salesman (of course) tried to get me to buy the box spring, to which I said no.  But now (about 2 months after buying) my wife and I feel some back discomfort in the mornings.  We've tried adding a pillow top to the already existing one, but it doesn't seem to help.  So I'm starting to wonder if I did, in fact, need to buy the box spring.  Do I really need the box spring?  If so, which one should I get for my particular mattress?

Gee, wish you'd checked out this site before spending your money...
That being said, your box springs(which typically aren't springs at all, merely an upholstered box) most likely is not your problem.  The pillowtop is.  If you took some time to read so many posts on this site, you'd understand what went wrong.  And, no, adding more padding will not help the problem, only make it worse.
In a nutshell, these bed manufacturers sell a nice feeling,cushy product in the stores, that basically is cheap polyurethane foam on top of some kind of springs.  The foam begins to break down under pressure(sleeping bodies) immediately, becoming noticeable sometimes in as little as two-three months.  Then you have lost your cushy support under the heavier parts of your body, letting them sink, which causes low back pain. 
I've been down that S&F bed route before. Mine lasted about that long.  Could not see the dents, but sure could feel them.  They finally sent someone out who was ill-trained(or perhaps well trained, depending on which side of the bed you are on)who didn't tell me anything, but in her report said that my bed was not supported by the frame as dictated in the warranty paperwork(which it WAS)so I had no claim.  Oh, and btw, the dent didn't measure 2" so that alone means no problem to them.  I gave the bed to a heavier friend so hopefully she could use it for awhile, but she tells me that it is a horrible bed and pains her every night.  Her camper mattess is better. 
I'm going to be money that by not buying their box springs you voided any warranty right there. 
Now for the good news....you can do mattress surgery yourself(possibly find a mattress maker who might consent to replacing your cheap foam with something better, hard to say)and replace that cheap foam with something more durable.  I have heard that S&F does have some nice springs under the foam, but by getting all that pillowtop comfort, you aren't sleeping the springs, but the foam itself. 
Again, wish you'd checked this site before spending the $, you might have saved yourself trouble and time.  Good luck!!!
Kait