I would put them directly at the sides of the listeners, as you described this would be about 10-15 inches from the back wall. This gives them some room for the tweeters to reflect off of the back wall, to give the additional diffused/spacious effect that they are famous for. Just as long as they aren't "in front" of the listeners.
As far as height, I think about 18 to 24 inches above the listening ear positions is good.
It's great to get into using the SPL meter, but you can always try your receivers setup and see how happy you are with it.
As far as the sub, I'm sure it would be best to upgrade down the road- but for now, try it both ways (M60s at small/80Hz xover, or full) and see which you prefer.
