LFE vs High-level subwoofer connection, for FIDELITY

I’ve tried both, high level and rca inputs. I only have one sub, so a bit of a challenge to blend for sure. I also have Maggie’s, so running both sub and mains on speaker level was difficult to blend the crossover point for the sub to disappear. On the other hand, taking a bit of a loss running through an avr, i have the benefit of using the built in crossover before it goes to the sub and the main amplifier. Well it may not Be perfect, my mains are not stressed, and it seems to blend pretty well especially using dsp and the callebrating room mic.

@blackbird55 Is correct IMO. LFE, Line-Level (XLR/RCA) and High-Level / Speaker Tap are different. So for Hi- Fi Stereo, If your only options are LFE and High-Level, go with High-Level.

FWIW I've used both successfully. A pair of REL T/9x using High-Level and now a pair of Arendal 1723 1S using Line-Level XLR. Much prefer the Arendal's for a host of reasons, but that's off-topic. They don't offer a High-Level option and I don't miss it at all.  

I use a Yamaha A6A AVR in my system which is set up for both home theater and two channel listening.  It’s geared probably 90% toward music, stereo only, and the rest the occasional home theater. The AVR is only used as a preamp in stereo mode as I have a separate two channel amp that drives my front left and right towers.

Per Yamaha‘s instructions I set my speakers to small which keeps the LFE channels engaged in stereo mode and sends the bass to the subwoofers depending on the crossover level that is set in the AVR. The Yamaha’s room correction set that up and I tweaked it a little bit. Currently they are crossed over at 80 Hz.  I run dual SVS SB 2000 subs. 

With this configuration, in my room and my system it is extremely musical. I cannot tell where the subs are at all they blend seamlessly with the primary L/R towers (Focal Aria 926s).  Generally I cannot tell where the towers are either unless there are some extreme left and right effects. The speakers just disappear into the soundstage as do the subwoofers and the soundstage extends well to the left and right and above and below the actual speakers. It is very enveloping.

Given my experience I feel it’s perfectly acceptable to run the subs through the LFE outputs. Between my manual tweaking and the room correction set up it blends everything very well. I would also think that you should be able to set a system up the same way as there’s nothing super special about my room but it is a matter of experimentation and tweaking. 
 

@larryincmh 

Per Yamaha‘s instructions I set my speakers to small which keeps the LFE channels engaged in stereo mode

I'm not sure you are aware but LFE downmixes stereo sources to mono. If your crossover is below 40Hz it doesn't make a difference because you cannot localize it. But over 40Hz you can start to localize it and you lose the dynamics of stereo if it's downmixed to mono. That's the main con vs high level.

@erik_squires 

For this reason, line level with a high pass filter is best, and I think this is how you get glorious subwoofer behavior

The KEF KC92 subwoofers are very sophisticated.  They include a variable frequency crossover adjusted via a dial on the back panel.  Furthermore, they can be fed stereo line-level input and return line-level to the main amplifier, after removing the lowest frequencies.  In other words, they can act as a high-pass filter.  The high pass filter also has adjustable frequencies, this time adjusted by DIP switches.  The line level signals use RCA connectors so balanced operation is not supported.

Note that these subwoofers can accept line level stereo from a preamplifier, or just Low Frequency Extension from a pre-processor.  Stereo uses the LFE input for one channel.  LFE is just a convenient label in this context.

The OP has an integrated amplifier and a passive pre-amplifier, which seems to duplicate the pre-amplifier function.  I am unclear how these are used / connected. 

Many integrated amplifiers allow an external processor to be connected between the pre-amplifier stage and the amplifier stage.  This is no doubt how KEF would prefer their KC92 subwoofers to be connected, and they would act as an external crossover relieving the amplifier and main speakers of some of the bass load.

However SoulNote are radical thinkers and their A2 V1 integrated amplifier A-2_BDA_EN.pdf eliminates as much internal wiring as possible.  They do not provide a pre-amplifier output.  On the other hand, the integrated amplifier can be configured as a pure amplifier, which may be how the OP uses it..

It seems to me that the OP is left with two sensible options.

Option 1.  Run duplicate loudspeaker cables to the subwoofers which means both main speakers and subwoofers will be presented with the full bass range

Option 2.  Use the separate preamplifier, and run the integrated just as a power amplifier, taking the high-pass line-level signal from the subwoofers to the power amplifier.  This relieves the amplifier and main speakers of low bass duties.  But the KEF subs do not support balanced connections, which is a shame in this configuration (theoretically at least).

I am a great fan of my SoulNote Equalizer but I do recognise that there are quirks in SoulNote's designs which make their A2 V1 integrated amplifier hard to integrate with subwoofers when used as an Integrated.  I am almost equally pleased with my KEF Reference 1 speakers which I use with a Velodyne sub with many configuration options.  The Velodyne has balanced input and output options. 

My KEF sub is a lesser model only used in my motorhome, and as an illustration of power cables making a difference!