I'd first find the anchor spot for your listening position, as a fundamental starting point, if it were my system. If you are a "sit-alone" listener/viewer, you have some choices to make in a perfectly symetrical room (in your case, side to side-back to front at least).
In a perfectly rectangular room, with one "centered chair", your challenges are: 1) getting out of all the "holes" in the response curve - 2)knocking down all the peaks as well! - 3) proper acoustical treatment for your room/setup - 4) deciding on whether you can get away with a "long-wall setup" for easier acoustical factors and likely better overall sound - 5)whether or not to go with a "centered listening position" setup or not!
For your room and description (not knowing all your equipment), these are the first factors that come to mind, you should adress - even if that means starting from scratch with your setup.
I mean, in a dedicated room, where you can move things around to optomize, you should! If it's not a dedicated space, with options to move things at will, your main goal is still flat response from all speakers from your chair(s), proper adressing of acoustical reflections from boundaries/walls/objects, etc, well placed speakers for a solid well spread image (in proximation to relative screen size, ifyou can), and yes, proper envelopement and steering. Also, proper phase from all speakers and sub(s) and seats. Room reverb you can't fix without serious acoustic treatments or room DSP correction.
I would first consider a 'long wall" setup for a "first try", as you will most definitely be able to achieve better sound, more easily - due to less first order reflection problems, a lack of acoustical treatment necessities, and closer proximity to your main speakers from your chair. I especially recommend this setup if you don't have money invested in acoustical treatments, and you are a single seat in the room - as opposed to the "short-wall" setup.
You can also get a bit wider with your speakers, and have more flexibility.
That all said, I HIGHLY RECOMMEND some sort of room EQ in ANY SYSTEM these days! small acoustical spaces need all the help they can get, to help balance out the sound. If you're not using any sort of DSP circuit/processor, I recommend the likes of the Rives Parc, high end EQ. All this stuff "adds up".
If you are doing the standard "short -wall" layout, looking down the long dimmension, I'd start with figuring out where you're gunna sit - yes - first. I recommend compromising center position seating in a perfectly symetrical space, because of the problems I mentioned. yes, you give up your center imaging with stereo listening, but you gain so much more with everything else! (infact, I will only do a center position when I can get flat response from my mains and sit where there's not "nulls" in the sound, mainly in "L shapped" rooms)
I'd start by using the 1/3 room dimmension for seating options, and 1/3 or 1/5th width spots for starters. If you want to keep your chair approximately where it is now, just near half mark, I suggest locating the 2/5th's or 3/5ths lenght spot(s), and try placing main l/r speakers in 1/10th length dimmension out from the front wall. Then, depending on where you have your seat from side to side, I recommend placing speakers where they have the overall flattest response in relation to the seat. - trying to balance flat response, proper soundstage width, and proper solidity of immaging. Center should be off of center width, and probably on a 20-24" stand under the image. If you have a perf screen pj setup, you have more options. This should however give you a solid response from the center (also make sure you have a rug between you and the speaker.
sides and rears, I must suggest tring to find locations where they are approximately where you want them (higher up for direct firing speakers, so they don't draw attention to themselves - pulling you away from the MAIN SOUNDSTAGE! (this is distracting, and highly anoying, IMO). You should be lost in a sounscape, not being bombarded with one speaker yelling in your ear!
I would then be finding where each speaker offers the flattest most solid response in relation to your ear position, yes, even the rears and sides! Each speaker should offer solid response and accurate reproduction, as that all adds up to or takes away from the overall sound, and causes tonality problems in the bass, among other problems.
If you can get your speakers placed around you for a coherent, well distributed soundstage, and get flat accurate response (It takes practice and experience - and a test cd and sound level meter are a must), properly deal with the acoustical relfection issues and balance the acoustical treatments for you size room (I recommed - in a shor wall setup - alternating absorption pannels with diffusion pannels, to the sides and behind mains speakers, and diffusion all around the back. In long-wall setup, you might forgot the acoustical treatments all together on sides!), and get proper phase at the critcal crossover from sub and speakers all around, you will have some dynamite sound! *(I notice your speakers are typical stereo speaker desings, with "open tweeter on top" desing. Make sure either sit closer to mains somehowe, or do ceiling acoustical treatments - so you get more direct sound vs. reflected)
All this helps add up to a solid, super dynamic, extremely coherent soundstage!!! fail to consider any and all, and the quality goes down ...it's that simple!
Not sure how this all comes across from how I've written it. But if any questions, let me know.