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[David Hoover]

I think the point was that the way probably the majority of people use a crossover or basic speaker processor would be with an input per channel, e.g. left and right in with left high, right high and either left and right low or a sum mono low out.  Any approach using a dedicated subwoofer feed would require a dedicated third input for the subwoofer signal and dedicated routing and processing of the subwoofer signal.  So for some people that may very well entail additional or different signal processing.
 
FWIW, high passing the mains at 80Hz and low passing the subs at 80Hz does not mean the resulting crossover is at 80Hz nor that it is flat.  The type and slope of the filters can affect that but probably more overlooked is that what really matters is not the filter frequency but rather the resulting acoustic crossover, in other words how the actual sound from the speakers interacts.  For one thing, having the same low and high frequency band signal levels out of the crossover does not inherently mean that the assocaited low and high band sound pressure levels are the same.  Or turn up the subwoofer levels and it is not just the subwoofer level increasing, it is also the resulting crossover frequency increasing.  It is not understanding some of these isues that I believe often causes less than optimal results with aux fed or any dedicated bus fed subwoofers.

This is true.  If you don't have 3 feeds, it's obviously harder to do.

I didn't think it needed that much detail, but thanks for adding!  Turning the aux feed up and down will affect everything in the signal path.  All the way from the aux feed to the speakers.  He's right - throw two different sets of speakers on the same crossover and amp setup and you will get two different interactions.  I disagree with it changing the crossover frequency of the subs if you crank them up.  All it does is increase the amplitude of the signal and bandpass section.  However, when you increase the amplitude, it increases all frequencies of that bandpass region.  The increase of all the frequencies in the bandpass gives you a different crossover interaction because the high end of that band pass region will be louder giving you a little bump above the crossover point.  However, it does not raise the crossover frequency point. 

[Mac Kerr]

I disagree with it changing the crossover frequency of the subs if you crank them up.  All it does is increase the amplitude of the signal and bandpass section.  However, when you increase the amplitude, it increases all frequencies of that bandpass region.  The increase of all the frequencies in the bandpass gives you a different crossover interaction because the high end of that band pass region will be louder giving you a little bump above the crossover point.  However, it does not raise the crossover frequency point.

You can disagree all you want, but you'll be wrong. Raising the level of the sub bandpass also raises the acoustic crossover frequency. Because the low pass filter of the sub, and the high pass filter of the lows have a slope, and are not brick wall filters, the point where they cross over will raise in frequency as the level of the subs increases. The electronic crossover is only theoretical specification, the actual important crossover between the 2 speakers happens in an acoustic space that depends on the level of the signal as well as where the electronic crossover is set.

Mac

[Brad Weber]

I disagree with it changing the crossover frequency of the subs if you crank them up.  All it does is increase the amplitude of the signal and bandpass section.  However, when you increase the amplitude, it increases all frequencies of that bandpass region.  The increase of all the frequencies in the bandpass gives you a different crossover interaction because the high end of that band pass region will be louder giving you a little bump above the crossover point.  However, it does not raise the crossover frequency point.

Look at Figure 7 in Section 4.3 of this, http://sound.westhost.com/bi-amp2.htm.  As the amplitude of the low frequency signal changes the corner points of the low pass and high pass filters do not change, however the crossover point where the LF and HF curves intersect does.  Even if the crossover provides a flat response, differences in the relative levels of the LF and HF output of the speakers can create the same effect in the resulting acoustic crossover.

[David Hoover]

I disagree with it changing the crossover frequency of the subs if you crank them up.  All it does is increase the amplitude of the signal and bandpass section.  However, when you increase the amplitude, it increases all frequencies of that bandpass region.  The increase of all the frequencies in the bandpass gives you a different crossover interaction because the high end of that band pass region will be louder giving you a little bump above the crossover point.  However, it does not raise the crossover frequency point.

Look at Figure 7 in Section 4.3 of this, http://sound.westhost.com/bi-amp2.htm.  As the amplitude of the low frequency signal changes the corner points of the low pass and high pass filters do not change, however the crossover point where the LF and HF curves intersect does.  Even if the crossover provides a flat response, differences in the relative levels of the LF and HF output of the speakers can create the same effect in the resulting acoustic crossover.

Exactly what I was trying to say! 

[Mac Kerr]

I disagree with it changing the crossover frequency of the subs if you crank them up.  All it does is increase the amplitude of the signal and bandpass section.  However, when you increase the amplitude, it increases all frequencies of that bandpass region.  The increase of all the frequencies in the bandpass gives you a different crossover interaction because the high end of that band pass region will be louder giving you a little bump above the crossover point.  However, it does not raise the crossover frequency point.

Look at Figure 7 in Section 4.3 of this, http://sound.westhost.com/bi-amp2.htm.  As the amplitude of the low frequency signal changes the corner points of the low pass and high pass filters do not change, however the crossover point where the LF and HF curves intersect does.  Even if the crossover provides a flat response, differences in the relative levels of the LF and HF output of the speakers can create the same effect in the resulting acoustic crossover.

Exactly what I was trying to say! 

How is that exactly what you were trying to say? As far as I can see it says exactly the opposite. What it demonstrates is that the crossover frequency changes when you change the level of one of the pass bands.

In modern DSP speaker processors the high pass and low pass filters of a "crossover" are often not the same frequency. They are just the frequencies that cause the acoustic crossover to be where the speaker designer wants it, and, and to have the response the designer intends. If the high pass and low pass are at different frequencies, what is the "crossover frequency"? It is the point where the response of the lower frequency driver and the high frequency driver cross. This point moves up or down as the level of either band changes. In the case of raising the level of the lower frequency section, the crossover frequency moves up. Even in a crossover where the high pass and low pass are the same, the same result occurs when the level of one of the bands changes.

Mac