ReL Subs: High Level or Low Level connection

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.

sns "I much prefer the REL’s integration vs the low level with DSP I’ve tried with other subs designed for low level outs."

What make and model of third party DSP or the make and model subwoofer with onboard low level input processing did you try?

If those "other subs" low frequency were rated at -3dB how did you determine their position within your room?

Low level only. Passive is best. 

I forgot to say why. An opinion should always be followed by a reason. Subwoofer drivers are powerful linear motors. They are also powerful linear generators and produce back EMF (electromotive force) This feeds into the output of the amplifier and effects everything else the amp is doing. Then there is the matter of an effective steep 2 way crossover. The low pass filter that comes with most subwoofers is like putting a marathon runner on crutches. The best they can do is  is generally 24 dB/oct. To keep the subwoofer out of the midrange these shallow slopes force the user to push the crossover so low that it is ineffective for most music and there is no benefit for the main amp and speakers. A subwoofer running up to 100 Hz has to be crossed at 48 dB/oct to keep it out of the midrange. Main amps and speakers benefit most from being crossed out at 80-100 Hz depending on the speaker. Two way crossovers are all low level. Digital is best.

Passive subwoofers have the potential to be superior to subs with plate amps. Closing an amplifier in a box is the antithesis of usual amp design with huge heat sinks or fans. Any amp run hard, including class D amps, are going to get hot. subwoofer drivers already have enough trouble dissipating heat. They do not need another heat producing device in their home. For a group of people who insist on putting their electronics on isolation platforms I find it interesting that they would except putting an amp in a vibration machine even though vibration does not affect electronics. Outboard amps are superior to plate amps. QSC makes the PL380, a glorious subwoofer amp at a reasonable price.