If the subs are properly dialed in with the rest of the system, they will have no affect on dialogue intelligibility. The only way they could is if you ran them really really hot.
Placing a sub a couple of inches from the backwall is not going to be of concern. Pulling them away from the corner does not necessarily make them weaker. It will simply change the response a bit. It could even change the response for the better.
If you want to know the best placements for your subs, you have to measure the responses for each given placement. If it were me, I would try to borrow a small sub that has a flat response, and use that to test different positions. Once you find the best spots, place the JTRs there. Sub crawl might be a good idea if you can move your couch and place the sub where you would normally sit spatially relative in room. You still need to measure the responses though.