Do you adjust your subwoofer

@mijostyn 

Yes Doppler Effect...  Very familiar with it.  While I really try to eliminate as much signal processing as possible, correcting response between drivers digitally, then via D to A conversion sending the signal via a proper crossover to the drivers I believe as you say can eliminate/minimize distortions in the music especially around the crossover points.  The full-range drivers are still somewhat trying to produce the lower frequency music while crossing over to the sub, but also sending just frequencies in the lower mid-range and up to the single-driver eliminates the low frequency intermodulation distortion of the mid-range /treble music from bass music to the full-range driver.  You've opened up a new approach I believe I'll try sometime soon with a rather larger capacitor to my full-range drivers in an attempt to pass lower mid-range and up frequencies only while letting the sub carry only those low frequencies.  I had already known of a greater propensity for intermodulation distortion on a full-range driver but didn't carry it to adding in a subwoofer and just sending the lower mid-range and treble to the full-range driver.  Great information !

@mijostyn 

Also, this makes me think that having stereo subs might be better warranted if I do the aforementioned changes...

@liquidsound 

You bet. IMHO 1 sub is a waste of time. I tell people to hold off until they can get two. No matter where you put one sub in a room you are going to get very lumpy bass as you move about the room and it will change with frequency. Within 6 feet you can go from too much bass to no bass. Two subs smooths this out and the more subs you add the smoother the response gets. I just went from four 12" drivers to eight 12" drivers. The larger the surface area used for low bass the lower the distortion, Getting down to 18 Hz flat is not easy. It requires a lot of driver and surface area. My system is up 6 dB at 20 Hz. It returns to 0 dB at 100 Hz. This gives the effect of a live performance at less than tinnitus inducing levels.

I use one sub in my room and it has worked fine for me. But again I use acoustic software that has RTA, phase , and coherence. If you are listening in one spot only, one should not do much adjustment unless you trying to fill an area that requires more area to fill like for HT where their is more then one person in the room then I can see your point in using an array configuration.

@mijostyn 

Where I have a small listening space, I have found that the small sub I have right now centered in front of my amp-stand sounds pretty seam-less with my stereo speakers.  I do want to send the full-range speakers just the upper-bass and up, which I'll experiment with in the near future...  Thanks again for your help on this !