LFE vs High-level subwoofer connection, for FIDELITY

@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!

@jbuddha882 

Also, has anyone heard that it is straining or damaging to a truly balanced, direct coupled amplifier to use high level inputs? 

I am interpreting this to be asking whether feeding loudspeaker output to subwoofers adds significantly to the load on the amplifier.

The answer is no.

The reason is that the impedance presented by the loudspeaker input at the subwoofer is very high so very little extra current is drawn.  The subwoofer would prefer a line-level input of around a couple of volts, and line level impedance is typically 48,000 Ohms versus a few Ohms for a loudspeaker.

The extra current needed to provide a signal to the amplifier(s) in the subwoofer is about 10,000 times less than needed to drive your main speakers.

In my post above, I suggested two options.  Only you are in a position to try them and the extra cables needed are inexpensive compared to the investments you have already made.

What do you think of your SoulNote?

@bartsw in regard to mixing down to mono, isn't bass mono and omnidirectional by nature, so bass should not be evident from a left channel or right channel anyway. It should just be sound pressure in a room.  If you can tell if bass is coming from a right channel specifically or a left channel, then your subs are set up wrong.  I have always been told you should never be able to hear a sub and pinpoint it in a room or it is set up wrong.

@jbuddha882 the answer to your question, whether feeding loudspeaker output to subwoofer adds significantly to the load on the amplifier.  NO, not if you're using a powered subwoofer connected to the high-level terminals. The amplifier is only sending the signal to the subwoofer if it's powered and not added power from the amplifier. Even still let's say you're using a not powered subwoofer, and you are still connecting it to the high-level terminals directly from the amplifiers speaker terminals you should still not have any issues with extra load on the amplifier because once you connect the subwoofer to the amplifier speaker terminals you then connect your stereo speakers to the left and right-out speaker terminals on the back of the subwoofer. Just understand that you must be using an amplifier that is capable of handling the powering of the subwoofer if it is a power-hungry subwoofer. You must also keep in mind that this also applies for if you are running power hungry stereo speakers such as 6 4 2 Ohm. If all 3 speakers including the subwoofer are 8 Ohm you can get by with a less wattage amplifier and will not put added strain on the amplifier and surly, not if your using high impedance speakers. 

@richardbrand  is also 100% right but he explains it in a much more TECHNICAL Manner with less wording than I. He is always very Technical but usually right in his answer.