I take it the 19', left side, is a window. If it were my room and I had no restrictions, I would most likely use that window for the speakers and my listening position would be with my back against the arches wall. I would then use some acoustic foam stuck to panels that would be hung over the windows when I'm listening. I suppose that's not practical though?
Firstly generally speaking, ceilings aren't a big problem as long as they're level and not sloped. It's floors, walls, wall openings, windows and doors, as well as the room contents that need to be looked at.
The first area to sort out is bass. A boomy or otherwise poor bass damages the higher frequencies so get this right and you are well on the way. After that deal with the mids and highs.
When you play a bass sound and walk away and towards the speaker you should hear the bass louder and softer. If you find a location that sounds good to you, that's where you ears should be. You can also try moving the speakers about a bit. As the room isn't symmetrical I wouldn't expect the ideal speaker positions to be symmetrical either. You most likely will need some bass traps but often you need to deal with specific frequencies. Usually that's related to room size but as yours is an awkward shape, measurements are the best way. A simple Radio Shack sound pressure level meter and test tone disc would be a fair guide. The Stereophile Test CD has some suitable test tones, as does their Test CD3.
http://ssl.blueearth.net/primedia/home.php?cat=2A search should yield some reviews of these discs.
Using more sophisticated measuring methods (like the REW:
http://www.hometheatershack.com/roomeq/) is better of course but require a lot of learning and more equipment. Similarly a digital equaliser can do this and help develop your sound but are also difficult to master (I use a Behringer DEQ2496).
Once the bass is to your satisfaction, or at least as good as it can be, you can listen to the other frequencies.
I agree about the coffee table. Sounds will reflect off that and to your ear and perhaps muddy what you hear.
I also feel uncomfortable with the left part of the sofa. Is it possible to remove that and just have a twin sofa facing the speakers? The removed piece of sofa may well absorb sounds on the left side. You also have the problem of sounds reflecting off the left window but on the right they are disappearing through the archway. Both these things affect the symmetry of the sound you are hearing. The solution for the windows reflections would perhaps be some mobile absorbing panels where the reflections occur (sound travels in straight lines like light so all you need to find the location is to have a friend hold a mirror by the window whilst you sit in your normal listening position and look for the speakers reflection - if you have no friends, you'll have to improvise!).
This site seems to have some good basic information:
http://www.crutchfield.com/S-kRtNmn7tJWj/learn/learningcenter/home/speakers_roomacoustics.html?page=3The main point is to take your time. It can be frustrating trying to perfect the sound but it is definitely worth it. If you now have the attitude that once the sound leaves the speakers, no matter how good your gear is, it will be affected by the room and its contents you are well on the way. All those ideas of tweaking the system with better cables or support cones etc. will pale into insignificance compared to what room treatment (and, I contend, a digital equalizer) can do.
