As I look at your photos, I see two places I like for testing out sub placement.
I don't think your Mains are quite full range enough to run as large in your AVR. (You are of course free to experiment and see what sounds best to you). Regardless, you should be able to do this with your Polk if you want, or wait til whichever you order comes in:
Check out the
Subwoofer crawl in this AH video. It is a crude technique, yet helpful and effective. In my room, I found a bad corner that not only amplified (what we call corner-loading) but also made the LF very boomy/muddy sounding. I found a spot where the LF just died completely. If you keep you mind and your ears open, you will start to hear differences as you crawl around listening at places your sub might go.
So the spots that I like? I would check the back right from your LP where you have that end table near your Dinette area. I would check front wall to the left in between your center and LMain. You can also try back left in the corner. This is speculation on my part, but I think getting at least one sub to fire into your LP from closer to the kitchen will help. That said, I'm also a big fan of placing the asymmetrically to maximize their effect on the room. Just sticking them up front or in corners does not guarantee you good Bass response.
Worth a shot, in my book to try the crawl and learn a little about how those LF waves behave!