13ft might be just far enough back, assuming the listeners in the front aren't reclined all of the time. Like others have said, it's worth a try and comes down to personal preference. You will definitely need to run Audyssey again though.
I tried raising my M80's before, but not on a subwoofer so I can't speak to how your subs will sound. I found that the soundstage was fine 16ft back but not as good 10ft back. Because my VP180 sits fairly low, pans across the front were not as seamless either. In the end, the difference in sound between my front and back rows was too significant for my liking. Besides, elevating the M80's terrified my wife; she was convinced someone was going to lose a limb, or worse.
