ReL Subs: High Level or Low Level connection

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.  

I run pairs of RELs in 4 out of my 6 systems. All connected via high level. Some with the upgraded bassline blue cables, others with the stock speakon cables.

I also run RCA’s for LFE to a pair of S510’s in my home theatre. I agree with other commenters above that it is worth a shot to try it both ways and see which you like better.

mijostyn   Your's was an informative ah-ha post for me.

Hacks that we are, despite the systems star grounding we assumed a grounding issue but were very hesitant to lift the amplifier ground.

The three of us could hear a noticeable issue when those high level cables were connected to the JC-1 amplifiers that were powering the system. It wasn't a huge issue but once we identified it became one of those things you couldn't help but hear.  

Mijostyn

Thank you for all that info.

Some a bit over my head.

Does what you say apply to SUBS with an amp in them ?

For instance my Rel S/510's?

Or just to passive subs?

Thank you

@oddiofyle

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. 

Does that mean you are running 'low level"'?