REL subwoofers have a unique connection system that they call Neutrik Speakon. It is advertised as, “This has the advantage of ensuring that the REL receives exactly the same signal as the main speakers. This means that the character of the bass from the main system is carried forward into the Sub-Bass System. This is a very important point and together with the REL’s Active Bass Controller (ABC), ensures superior system integration of the Sub-Bass System with the main system.” While it is true that this type of connection will not interfere with the signal going to the main speakers I have found that this type of connection is not a superior way to integrate a sub-bass system with the main system.
The problem is that you will almost always get elevated bass response where the sub-bass system and the main speakers overlap in response. I find this problem to exist in every REL setup that uses a Neutrik connector that I have encountered. This is caused by the additive effect of the the two systems. Other methods use a combination of low and high pass filters to avoid the problem. This type of connection is also found on the REL subwoofers which is the proper way to integrate a subwoofer into a stereo system. The following is an illustration of the Neutrik connector.
To summarize, I recommend avoiding the Neutrik connector system found in REL subwoofers. I would either use the bass management found in a surround processor or the high and low pass filter system found in the REL subwoofer itself.