I tried it back when I initially set up my PC13Ultra and found that it made no difference no matter what I set it at. Considering how important the room is to how a sub is going to perform, I wasn't surprised. My room was in bad shape. Since then, I rearranged my room and added bass traps which smoothed out the bass, but haven't moved the Room Comp from the Off setting. (In fact I forgot about that adjustment until I read your post.) I might play with it again to see if it will improve/degrade the sound.
The Room Comp is essentially a selectable high pass filter, used as Mike said to limit low end room gain. I had it set to Bypass on mine when I had my Ultra behind the seating area.
Recently though, I moved the sub to the front of the room, and now the room adds a hefty bump in the 16-17hz range as measured at the seating area. Turning the Room Comp to Large took off about 2 db of that hump and smoothed out the remainder of the response below that point, until it finally rolls off gently. It is both a measurable and quite audible improvement. I'm a believer now. :yes:
Just for grins I tried the Med and Small settings, and while they trimmed that hump even flatter, they also caused a much steeper and sooner low end rolloff below that hump. Too steep and too soon for my tastes.
More than likely I didn't notice any difference with the settings because my room and placement was wrong initially. I'd say it was an adjustment you would use in the final stages of setting up the sub to get closer to perfection. Now I'm going to have to break out the Rives CD and RS meter again just to make sure. Thanks guys. (Football is almost over, gotta have something to do on the weekend)