Will a subwoofer add depth and clarity to my system, or just bass?

Jetter, I don't know how to reliably predict in advance what would be the ideal, so some trial-and-error is probably called for.   You might even find that something in between, like 45 and 135 degrees, works best.  Or you may find that it makes little difference.   And it doesn't have to be 90 degrees on the dot.  I've gone a bit over 90 at times. 

Phase quadrature will result in something closer to semi-random-phase summing of the outputs of the two subs in-room, which implies partial cancellation, so you might need to increase the level of the two subs just a little bit.   

Is "Jetter" said with a French pronunciation? 

Duke

The ear simply does not have the time-domain resolution to hear a slight timing mis-match in the subwoofer region. The ear is UNABLE to even detect the presence of bass energy from less than one full wavelength, compared to which small timing errors are insignificant. What the ear IS very good at, is detecting SPL differences in the bass region. This is implied by the bunching up of equal-loudness curves south of 100 Hz. So the ear can indeed hear something going on as you fine-tune the phase control, but what it’s hearing is the effect on the frequency response.

If the main speakers are a bit muddy in the upper bass, you can have the subs overlap them a bit and then use the phase control to dial in some cancellation, thus improving the clarity.

I’ve had many customers claim to hear an improvement in clarity from the addition of subs without highpass filtering the mains, and I’ve heard it too, but cannot explain it, apart from the occasional situation described in the preceding paragraph. Of course there is also the improvement in clarity that can come from highpass-filtering the main speakers to reduce cone excursion, in which case the transparency of the highpass filter can be a factor.

Regarding adding a sense of depth or sense of envelopment/immersion, if you have two subs, you can place one at either side of the listening position and dial in 90 degrees of phase difference. This will tend to synthesize hall ambience. Credit to David Griesinger for this idea. The 90 degree phase difference ("phase quadrature") will also tend to improve the modal smoothing.

Sometimes adding a subwoofer to a tonally well-balanced system tips the spectral balance to the dark side.  When that happens, we can bring balance to the force.  We can add a rear-firing tweeter to just nudge the spectral balance back to normal, without messing up the imaging by adding another source of first-arrival sound. 

Duke

@avanti1960 wrote: "Adjusting the continuous phase control provides much more than a slight amount of timing adjustment and my ears were quite capable of hearing when I had the correct phase setting."

And I totally believe that you are quite capable of hearing when you had the correct phase setting. I just think you are most likely hearing the frequency response improvement that arises from your phase optimization. Whatever the case may be, thank you for throwing a spotlight on the improvements possible from adjusting the phase control.

Duke

Millercarbon, my suggestion is this:  Once you have placed the subs, first adjust the gain, then the frequency, then the phase on the sub amp.   Expect to cycle through gain-frequency-phase several times.  

The word "timing" seems to me to imply "arrival time".  The ear/brain system is not very sensitive to arrival time in the bass region, but it is sensitive to decay time, because decay time translates to frequency response in the bass region.  Let me explain:

Because speakers + room = a minimum-phase system at low frequencies, the frequency response and the time-domain response track one another.  When you have a time-domain problem, you either have a peak or a dip.  The good news is, when you fix one, you simultaneously fix the other.   So bass traps reduce the decay time and thereby fix the frequency response along the way.  Likewise when a distributed multisub system fixes the frequency response, it also fixes the time domain response along the way.  Peaks take longer to decay into inaudibility than the rest of the signal, which is why they make the bass sound "slow". 

Back to "arrival time" for a minute.  I read an AES paper several years ago where they played signals of less than one wavelength at bass frequencies through headphones.  The listeners did not detect any sound.  They could not detect the presence of bass from less than one wavelength, and had to hear multiple wavelengths before they could detect pitch.  This implies that relatively small timing differences - "small" relative to the wavelengths in the bass region - are not going to make an audible difference in and of themselves. 

But in the bass region, the ear actually has a heightened sensitivity to differences in sound pressure level.  This is shown by the way equal-loudness curves bunch up south of 100 Hz.   A 6 dB difference at 40 Hz is subjectively comparable a 10 dB difference at 1 kHz.  This is why we can hear the bass so much better when we turn the volume up.  This is also why it takes so long to dial in the correct gain setting on a subwoofer amp - if we're off just a little bit, it's distracting.  Finally this implies that the subjective improvements from smoothing the response in the bass region are greater than we would think simply from eyeballing before-and-after curves.

So I think that where the phase control makes an audibly significant difference ends up being in the frequency response domain, especially in the region where the sub is blending with the mains.  I believe that the correct phase control setting is the one that results in the smoothest frequency response, but this is found in conjunction with the gain and frequency controls.  They all work together... gain makes the biggest difference so we adjust that first, then we adjust the lowpass filter frequency, and then the phase, and then we cycle back through fine-tuning at least once.

Imo, ime, ymmv, etc. 

Duke