Subwoofer Question

If you can 'locate' the single sub easily, then of course it affects the imaging.  Even if it does not technically 'pull the image' to one side, the ears  ability to locate the bass affects the image in your brain.  If properly set up, a single sub can often still easily be located.  Just close your eyes and listen.  If you can point right to the sub, it it not nearly as ideal as two subs.  In both of my listening spaces, my pairs of subs are located well off the the sides and are not even easily seen from my listening positions. Placing them here made a huge difference compared to right beside my speakers, but it was for decreasing booming not imaging issues.  Congrats on turning the lowpass filter down to help!

yes harmonics in the airwaves are captured electronically during recording, any sound will be, and the crossover will subsequently send different frequencies to different drivers

but whatever is vibrating is what creates BOTH the fundamentals and harmonics, and, when you re-create an electronic signal into sound, you make something vibrate,

and

the thing (any thing) that vibrates creates both fundamentals and simultaneously creates a new set of harmonics, again, each time,

look at the patterns created by harmonics in the sand on the flat stiff steel plates. (far stiffer than any lightweight beryllium coated stiffened cone).

@elliottbnewcombjr 

the thing (any thing) that vibrates creates both fundamentals and simultaneously creates a new set of harmonics, again, each time

Not if it vibrates as a pure sine wave, when there are no harmonics.  You would be better off looking at Fourier analysis which shows that any repeating waveform can be represented by a set of sine waves - the fundamental and the odd harmonics.

Strangely, audiophiles tend to inhabit the frequency domain and talk about bass, midrange and treble, whereas in real life sound exists in the time domain which gives rise to discussions about Prat.

You can mathematically convert that repeating waveform into a set of frequencies using a Fourier transform.  You can also take the frequency set and convert it back to (almost) the original waveform.

There is an unfortunate glitch as the original waveform approaches a square wave in form.  The reconstructed waveform has a sharp spike surrounded by ripples.  You can see these as fringes if you look at sharp edges in a magnified digital photograph from a compressed file.

Huh?

No. But unless the subwoofer is next to you, you're best bet is a second matching sub placed elsewhere to offset room modes and standing waves. You don't want to be sitting in a peak or null.