Use the unused speaker terminals...the REL presents 100,000 ohms or something to the amp so you can run it as an "extra" speaker and it will get the same signal as the main speakers (since it's the same amp...sort of the point) simply from another output, and not bother the amp. Also, I don't see any reason why you can't run a REL by taking a signal from the speaker terminals the amp is already driving (may seem weird weird but makes sense...take one plus side from each already hooked up speaker and a negative from one and yer in business, albeit with strange wiring) as it results in the same thing. Also, Neutrik plugs are really easy to wire up for RELs and the proper hook-up is on the REL site.
Connecting an REL sub to a Linn Classik
Could this be done well by terminating the REL cable with the Linn-style plugs on the amp side and plugging into the unused(speaker 2)terminals? I ask because the REL manual really stresses using the SAME terminals your speakers are connected to, but is this that critical? Short of connecting both the REL cable and speaker cables to common plugs, I can't think of another way to do it. Thanks.