While I cannot comment on the overall sound quality of your entry level(ish) Onkyo AV receiver (my past experiences have been less than satisfactory, personally - I hope yours is better). I can say that you will most DEFINITELY need to run those large speakers as SMALL!!! (80hz crossover).
I do 100% garantee that recever does NOT have enough power to properly drive those full range speakers - especially full range! Running them as small/80hz will take much of the power load dubties off the receiver, and free up some dynamic room in the rest of the frequency range. You will then let the powered sub handle the bass dubties (yes, even though those towers have a couple/few woofers, you'll get much much better dynamic response from them if you let the sub do the hard dynamic work down bellow...trust me on this from over 15 years professional experience). With your lowish current receiver trying to drive all those speakers - especially full range - your challenge is "efficiency" (or lack of) in your system/room. In short, you need all the help you can get (infact, getting an outboard powered multi-channel amp down the line would be a great benefit, if you have pre-out's for your channels...do you?)
A benefit you do have is that your speakers are Dappolito configured.(woofer over tweeter over woofer) Even without knowing the crossover on those, you'll likely be getting much help in reducing/controlling the ceiling and floor reflection interaction with your ears, which otherwise would always smear the immaging, reduce dynamics and detail, etc. So, your speaker design helps, yes. So that's a plus.
The Polks also likely have a smooth, non-fatiguing sound, which is probably just fine - and I do like some of the Polk speakers I've heard over the years. So I hope they sound good to you too.
Another issue is your setup with speakers on one side of the room, and your ears right up against the wall on the opposite! YOu would do MUCH better with lots of acoustical and sonic issues if you could get your main listening location out from the back wall! This is certainly not a good location, acoustically. Yes, the Audyssey will help knock down at least some of your peaks, if not all (you have the basic Audyssey version) - however it will not deal with reflections off the wall back to your ears, combfiltering, etc.
Also, are you good with the size of that small image/screen from as far back as you sit, seeing that you are into movies? May I suggest you consider even a removable 105-120" PJ screen and entry level LCD/DLP projection system as an alternate for doing your movie dubties in a controlled lighting environement?! for around $2500 new you could get a superb hi-def movie experience which will be night and day better for movies than your paultry 42" image from across a 20' span in that room! It's a thought, anway.
So, to recap, at minimum, I would recommend setting your receiver to 80hz croossover, likely putting the subwoofer near the middle/front of your room, just off center for best phase response, bass reinforcement, and integration with your mains. (although you WOULD get more bass reinforcement in a corner, which might work out better for you with the Audyssey - so try and see).
I would also not put your main speakers right up against the front wall, but out a couple feet, different distances from the side-walls than the front, and maybe toe in a pinch.(that's a "quicky" suggestion)
Yes, I would consider moving your back seat to at least 1/5 or 1/7th of the room length dimmnension, if not 2/5th's or 2/7th's. Actually, 1/3 would be even better - but it's always a setup/asthetic comprmise with your life-style choices, usually - but not the back wall location...ick!
Also, I suggest a few diffuser/absorbres between your side-walls and your listening seats. Home made options are possibly cheaper, maybe a bit more work, unless you simply stick some un-even surfaced furnishings in those locations, instead, as alternatives - just a suggestion.
And, yes, if your receiver has pre-out's for an amp, I'd STRONLY suggest getting some 5, 6 or 7 channel outboard amp (Rotel, Adcom, Parasoun, ATI, etc, WILL ALL BE 200% SONIC UPGRADES TO YOUR RECEIVERS AMPS...NO JOKE!)
So, since I'm all about quality sound and image experience here, these suggestions will definitely get you a better audio/visual experience if you heed them.
Hope this helps.