My Quad 63 ESL's are at least 13' from the rear wall because my system is in the basement where I can do a lot positioning experiments without being scolded by the wife. I moved everything to the basement a few years ago because I know how important the environment is in one's system.
The Power of Positioning
I moved my Sonus Faber EA 11's out from the rear wall in my listening room. They were closer to two feet from the rear and I brought them out an additional foot to three feet from the rear wall. Well let me tell you the improvement was fantastic. The upgrade in sound was greater than any equipment upgrade I have ever made. I am now a believer in proper setup. Work on your room and positioning and you will be rewarded.