Well, if you only really care about one seating position (the "sweet spot" as it were), the first thing to try is just moving your seat. If the bass is loud enough that it's actually hurting your ears, there's no question that you are sitting right on a bass node peak - the point where sound waves from your speakers are reflecting off of your walls/ceiling/floor and "bouncing" back and forth and you happen to be sitting right where they overlap and combine to create a huge swell in the volume! If you have your seat pushed right up against the wall behind you, pull it away from the wall by 1.5-3 feet. If you're lucky, you just happen to be sitting in one, big peak and moving the seat will get you out of it.
My own theatre room is a very similar size and I've stuck a lot of absorption in there to quell the bass reflections. I'm afraid you just can't cheat physics! Bass is going to bounce around in your room and create peaks and nulls - that's just the way it is.
Things get more complicated when you have multiple sources of bass, but it can also be the solution - it's just not easy to know exactly where to place them! Right now, you've got two speakers making bass waves and then the room's dimensions are creating standing waves. If you correctly place multiple subwoofers, you can break up those standing bass waves, but you can't just place subwoofers willy-nilly. You COULD do it through trial and error (crawling for bass would be the fastest way to get a rough idea of where your subwoofers might go in order to help), but what you'd really want are some pretty thorough measurements.
If you're not willing to use thick absorption panels on the walls, about your only other option is to stick as many sound absorbing pieces of furniture in your room as you can. Gik Acoustics sells end tables and pedestals with thick, bass absorbing insulation built in. That would be one option to "hide" your acoustic treatments in pieces of furniture that you might need anyway.
