LFE vs High-level subwoofer connection, for FIDELITY

@jbuddha882 

A lot of the advice you are getting is generic and in my opinion should not be applied to your quality, powered subwoofers which contain 1000-Watt amplifiers and active crossovers.  Remember that KEF is probably the most scientific of the English loudspeaker manufacturers, and is also a leading exponent of active (powered) loudspeakers.

1. The provision that lets subwoofers take high-level input is just a manufacturers workaround for situations where line-level is not available, such as integrated amplifiers that don’t provide pre-out as a line level.  Yours is an expensive example of such an integrated amplifier, and has omitted pre-outs for signal purity reasons.
2. High-level (or loudspeaker level) is inferior to line-level because it includes the distortions of the power amplifier and the back voltages generated by dynamic speakers.  Inside the subwoofer, it is reduced to line level anyway.
3. Most subwoofers are manufactured to fit budgets which can only afford one extra box.  They therefore take both left and right channels and sum over both to get the bass signal, giving rise to the idea of ’mono’ bass.

Your excellent KEF KC92 subs are actually bipoles - they each contain two subwoofers and two amplifiers acting at 180-degrees to each other.  Because you have a pair of KC92s, you actually have four sources of sound.  There is no benefit in stacking them.  Instead try them close to your main speakers and feed each sub with the relevant left channel or the right channel, but not both!

My recommendation for line-level hook-up is to use RCA-terminated interconnect cables (IC).  Connect your passive pre-amp output direct to your subs, left to left-channel input for the left sub and right to left-channel input of the right sub.  (When using KEF subs for a single channel, always use the left sub channel).  Then connect the left sub left-output to your power amp left-input, and the right sub left-output to your power amp right-input.  Make sure your SoulNote is switched to power amp mode.  Then you can play with sub volume to match the speaker volume, and the two crossovers on each sub to get the sound to integrate pleasingly.

If you want to just try the high-level path, you lose the benefits of the active crossovers built into the KC92.  I would locate each sub close to a main speaker and run a short pair of insulated wires from the speaker binding posts to the sub’s speaker-in left channel.  These extra wires hardly carry any current, so can be as thin as interconnect wire.  Ideally they would be gently twisted.  The proverbial lamp wire would get you started

+1 @phusis ​​​​@richardbrand 

@bartsw 

"I'd push back and ask why would you want to listen to bass in mono? Mono is so obsolete."

You can push all you like. We can disagree on audio setups, but there is no reason to be disrespectful. I mentioned subs, not mono.

I have borrowed this from @richardbrand who correctly states:

2. High-level (or loudspeaker level) is inferior to line-level because it includes the distortions of the power amplifier and the back voltages generated by dynamic speakers.  Inside the subwoofer, it is reduced to line level anyway.

You misread and misunderstood my post. Well done. Carry on.

@gournard 

You can push all you like. We can disagree on audio setups, but there is no reason to be disrespectful. I mentioned subs, not mono.

Trust me, I wasn't being disrespectful. Being disrespectful reads 100x worse. If you can't question something that doesn't make sense to you, then there's something wrong here.

High-level (or loudspeaker level) is inferior to line-level because it includes the distortions of the power amplifier and the back voltages generated by dynamic speakers.

But that implies using a speaker (tweeter, mid-range and woofer) is inferior, especially when using a Hypex, Purifi, ICE class D amp that measures better than most 'no-name' amps found in subs. Yet, we choose not to listen to music with just subwoofers instead of speakers.

It also implies paying up to $$$ more for 1000 feet of RCA cables that are subject to EMI/RFI and running them all over the room is superior to running a 3 feet lamp cord to a speaker, which isn't subject to EMI/RFI within the audible range. There isn't a one is better than the other here. I've compared stereo line level from my streamer to my subs vs high level and there's no real difference other than cleanliness, convenience and cost.

I'm using both LFE and high level with class D gear, with and without tubes in the chain, and see a lot of nonsense opinions based on hearsay, not self-experienced use, like the flat measurement fans. Room correction is blind to overlapping frequencies. It simply sees peaks and dips and corrects it regardless of high level or line level, just like it does on tweeters and mid-ranges. Also, tube amplifiers objectively measure worse than class D. But is is not fact because tube fans believe it is subjectively superior to class D. Some people like the bloom in bass, especially for certain music.

@bartsw 

Yet, we choose not to listen to music with just subwoofers instead of speakers.  ...  It also implies paying up to $$$ more for 1000 feet of RCA cables that are subject to EMI/RFI and running them all over the room is superior to running a 3 feet lamp cord to a speaker, which isn't subject to EMI/RFI within the audible range. There isn't a one is better than the other here

Hard to know where to start on this twaddle!  If a subwoofer could be made to handle the entire audible frequency range, that would be excellent.  But that would turn if from a subwoofer into a full-range driver. Physics stands in the way, at least for conventional dynamic drivers fabricated from cones and electromagnets.

Quad has made full-range drivers for over 40 years, but they are electrostatic panel speakers using featherweight membranes.  They also use sophisticated delay lines to emulate the ideal of a point source of sound.

Other manufacturers like KEF and Fyne address the point source of sound ideal, using dynamic drivers with the tweeters concentrically placed inside their woofers.

With my preferred wiring suggestion, the OP would only need 1000 feet of RCA cable if his electronics were 250 feet from his subs.  Unlikely.  At a separation of 6 feet, he would need a more reasonable 24 feet, one fortieth of what you suggest. 

As far as EMI is concerned, I would prefer to run balanced interconnect cables but KEF has made no provision for balanced connections.  Presumably they don't see sufficient benefit.  Note that EMI covers the entire electromagnetic spectrum, of which RFI is just a small part.

The OP's SoulNote amplifier is deliberately designed without negative feedback.  This means it measures poorly in conventional frequency-based distortion tests.  It is done to get the highest possible fidelity to the transient waveform in the time domain, which is where music actually resides.

The OP can easily try both wiring options to see which is better for the FIDELITY in his thread title.