ReL Subs: High Level or Low Level connection

After 50 years selling (beginning with the original M&K subs in 1974), designing, building, and using subwoofers in both home and professional live music here's what I know about connecting subwoofers.  There are no advantages to using high (speaker) level connections except convenience or accommodating equipment without low (preamp) level connections. The only correct method of connecting a subwoofer is via a high pass filter to the mains and low password filter to the sub(s). The HP/LP configuration minimizes LF overload to your mains reducing intermodulation distortion from the LF content and improves system dynamic range by as much as 6dB - the equivalent of quadrupling your main amp power. Likewise the LP filter removes higher frequency content from the sub(s) and with it the coloration associated with sub trying to do the mains job. Any HF content from a sub muddies the lower midrange and adversely impacts an accurate soundstage and imaging by creating a third ill-placed source for both channels lower midrange content. While other suboptimal connections may function, they are compromised in one or more aspects and never desirable. Placement of the HP/LP filter may be be in the increasingly common LFE preamp outputs, a separate component, or in the sub itself. In the latter case, longer interconnects are typically required to enable proper sub placement both to and from, so this is less desirable due to potential long cable issues. Crossover frequency and slope is largely situational, but in general keeping crossover frequency at 80 Hz or less is desirable as are steeper 12/18/24 dB per octave crossover slopes. Finally, placement is also largely situational although avoiding known room node points (e.g. halfway down a room's length) is a good starting point. 

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. 

I have two S/510.  For two channel, they are connected via high level with Analysis Plus wires.  I also do home theatre via a pass through on my preamp. I'm connected to the LFE input from my surround processor.  Seems to work well.

I want my mains to run full range for two channel. 

High level is what REL's designed for, and my experience this best. I've experimented and continue to experiment with high level cables, thus far stock, Duelund tin plated copper, always Neutrik connectors not the cheapo copies. Both stock and Duelund litz wire, next will be VH Audio UniCrystal Airlock, solid core wire. Also various configurations of hookup to my 300B monoblocks vs 845 stereo amps. Finally power cords.

So many variables, not to mention crossover and level on subs, almost forgot to include running on same dedicated circuit as amps with added outlet. Some complain about lack of adjustment with REL's, that's not me. I much prefer the REL's integration vs the low level with DSP I've tried with other subs designed for low level outs.

I connect using high level with Neutrik SpeakOn connectors which are recommended by REL, never look back.