ReL Subs: High Level or Low Level connection

For me hi level is best. I’m sure there are advantages for low level with some systems. I have two EVO 400 amps plus 2 REL 212SX. My main speakers are connected to the 4 ohm tap and the RELs to the 8. It is an ideal situation in my opinion. The hi-level adjustments are easy to integrate with the main speakers. 

I like to run my mains full range, never with any type of high pass going to my Amp.  With a low crossover the REL will never call attention to itself.   

I start with RELs recommended volume initially and the crossover set to 25 and slowly work my way up with a bunch of different material until the sub disappears and does no harm, only making itself known when the content demands it. 

@thebingster I have both subs set somewhere around 60hz. I had them set too low previously and could hear the transition between my mains and the subs. I have stand mounts so that might make the transition more obvious when it’s not right but I could clearly hear it. I tuned it by ear by raising the crossover until the transition was smooth and seamless. What the correct crossover point for you is really speaker and room dependent so start somewhere just above the lowest frequency your speakers are rated and work your way up until the integration works for your ears.

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.