Good idea actually. From what you describe it does sound like the rears are more likely the cause of problems, take them out and run with just the front three and see if the problem goes away.
If it does, then the rears are the problem. If that's the case, do the reconnection stuff we've discussed already, and if it's still a problem then -as already mentioned - it could be the wires or the speakers. Difficult to try 'new' speaker cable with such long lengths but it might be worthwhile trying the rears with long runs of cheap two-core electrical cable to rule out an internal short on one of the speaker cable lengths. If that still doesn't work, you might be stuck with taking the back panels off the rears and checking for internal shorts there; for example, my 9.1s have internal connectors that can loosen and rotate internally, touching and causing a short.