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.