LFE vs High-level subwoofer connection, for FIDELITY

Wow, why is it that subs are the most misunderstood aspect of achieving good sound. Subs? Can't do without them! As for "if the speakers bass goes down to 50 Hz then set the subs XO to 50 Hz" No, just no. You can not without measurement know what the speaker/room interface looks like, so if your 50Hz is exciting a room mode right at your listening position your subs 50Hz will be adding more. Expect a large unwanted peak. Conversely, you could be sitting in a null and will need help from the sub.

Forget all that, including the mindless sub crawl looking for the strongest bass. OK so you find it but what about the null that might be sitting close by, how would you know? It,s a null right? A null has no sound, it's a cancellation of bass frequencies meeting out of phase and if by some magic you could locate the null what are you going to do about it?

As I mentioned, do not worry about all this just get a mic. and REW and adjust the subs for t5he flattest and smoothest response. That's it. And please use line level guys. Why would you want to hear the, generally speaking, higher distortion of the power amp and if it's tube we all know that the output transformer saturates easily at the lowest frequencies. Now why would you wanna hear that? Some here like to just splab whatever goes without thinking it through.

Some tips from vast experience: Choose sealed box subs and they must have variable phase. I no longer set up systems if the subs only have 0-180 phase flip like the dinosaur REL. With only a polarity switch for phase you will chase the subs all around the room. With variable phase you can place the subs just where you would like them except symmetrically.  I say again: use the clean line level signal to drive the subs. 

@thehybridvigor 

High level is fine if you have no intention of digitally altering the signal (room correction, EQ, delay, phase adjustment) on a separate device.

Sorry I don’t understand; why can’t you digitally alter the signal with room correction using high level?

@gournard 

And please use line level guys. Why would you want to hear the, generally speaking, higher distortion of the power amp and if it's tube we all know that the output transformer saturates easily at the lowest frequencies. 

The additional noise and distortion is a non-audible issue except with tubes. People argue, why buy tube gear in the first place? Yet, they do because they value the pros over the cons. I'd push back and ask why would you want to listen to bass in mono? Mono is so obsolete.

"You don’t need additional processing when daisy chaining subs."

It seems to me that if you daisy chain subs you will create peaks and troughs depending on their distance from each other and additional processing could help fix this. 

@jbuddha882 wrote:

Grok Ai states it [i.e.: high-level connection] matches your speakers tonal balance, timing cues, and sonic signature, because it shares the same signal path through the amplifier that your speakers sound travels through. It’s also a great way to get tube or amp sound into your bass region.

High-level connection aims at addressing the importance of the chosen amp for the subs and how it correlates with the mains, meaning in this case at least "borrowing" or rather emulating the signature of the same amp that is used for the mains over to the subs and its own built-in amp. 

However why not do it properly if the situation allows, or you'll accommodate it to do so: configure a sub solution outboard actively and acquire an additional amp similar to the one you're using with your mains + a quality DSP, then high-pass your mains to relieve them of LF and low-pass your subs accordingly.

With a quality DSP you'll be able to experiment with fine-tuning the exact crossover point between the mains and subs down to single Hz increments, a wide range of filter slopes and types, using overlap or not, fine-tuning delay in small steps/ms, gain adjustment down to 0.1dB steps, etc. 

Preferably you'll do the above in a fully active setup, and maybe you'll realize that using the exact same amps for the mains and subs isn't necessarily the best solution - certainly that the choice of amp used for the subs isn't trivial at all.

Indeed, the rabbit hole goes much deeper than simply latching on a sub or two with its own built-in amp to a pair of main speakers run full-range. Instead treat it like you would a complete speaker system with measured, deliberate choices top to bottom with amps and all, and not just an afterthought. 

@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