Those are some really tight room dimensions! You are running some really great components and speakers. Since you mentioned that the room is dedicated I think you would get a lot of benefit out of some acoustic treatments: bass traps in corners and absorbers on first reflection points.

Do you run the 803s full range? Can you / have you tried feeding the LFE to the L/R mains + sub?

The X13 is an awesome sub. You would really need to work on placement given those room dimensions. Two X12s would offer a lot of bass smoothing benefit, but you'll be really crowded to get the placement right. Some bass traps would really change the way the room influences your current kit and make a single subwoofer (and the full range L/R pair) perform better.