I have 2 RELs and I've tried both connections with a couple of different amps. High-level is the way to go IMO.
That way the RELs are fed exactly the same signal as your mains rather than a filtered signal.
If you go with Speakon connections I highly recommend you upgrade the stock cables. Frank at Signal Cable makes a high quality custom cable for around 1/2 the price of the REL Bassline Blue cables.