Floor-standers? Stand-mounts? How big? Horns? Ribbons? Conventional drivers? Bi-pole? Di-pole? Right off of the top of my head, I'd say if these are conventional dome/cone drivers in a box speaker, you're too far away if they're only 8' apart. Are they toed in at all? If yes, that will make matters worse in the above scenario. At 8' apart, you need to start at 8' away, with the speakers toed in so you can see just a bit of the inside plane of each speaker box, then play with it from there. At just 20" from the side walls, you may need to dampen the first reflections with something. Here's the bone-simple version: Sit where you're going to sit. Have a friend with a mirror start on the wall right next to the speaker, then bring the mirror forward, along the wall, until you see the reflection of the speaker in the mirror. Put something soft there.
If you're stuck with 8' apart and 13' away, bring those speakers in another 6" each, eliminate any toe-in, dampen the first reflections (wall haning, potted plant, case full of books, soft chair - anything soft and diffusive will treat if not cure) and see what you get. You're going to get compromise, no question about it. What you gain in breadth you may lose in precise imaging. Or maybe not. Depends on what the answers are to all those questions up top.
Tim