Do you adjust your subwoofer

@liquidsound 

You are on the right path. Might I suggest a second subwoofer and a complete two way crossover so you can put a high pass filter on your main speakers. This will lower distortion and increase headroom dramatically. This is just, if not more important than the added bass a subs brings to the table. This is especially true with full range drivers. The second subwoofer will smooth the bass response throughout the room a little better. Subs perform best up against the wall and in corners, but in order to time align them with the main speakers you need digital bass management. 

@mijostyn 

Thanks for your suggestions...  I have thought about adding a second sub for a more balanced 2-channel sound (cutting the volume by ~3db...).  I have a rather small listening-space (not really a room).  And space is at a premium, especially for sub placement.  I tried several positions with the sub including corners until I found that placing it directly in front of the amp rack sounded the most seamless and balanced.  Since my speakers are single driver and crossover-less, I didn't want to add in any crossover just letting the internal sub adjustments handle the crossover slope/frequency/volume...  I wouldn't mind trying some of your suggestions in the future as I do enjoy tweaking my system.  I just auditioned and bought Iconoclast interconnects/speaker cables which are a notable improvement in my enjoyment.  

Thanks again !  

@liquidsound 

That is probably where the sub is best time aligned with your main speakers. With digital bass management you can put the subs anywhere they produce the best bass and time align them digitally. 

 I also use full range drivers (ESLs). Are you familiar with Doppler Distortion? If a car passes you with the driver leaning on the horn just as the car passes you the horn changes pitch. Unless there is a large capacitor on your full range drivers they are still moving to low frequencies even if they can not project them. They are moving towards you then away from you and just like the car they are Doppler Distorting all other music they are carrying which for people like us with full range drivers is EVERYTHING! Putting a high pass filter (the other 1/2 of a two way crossover) on your full range driver lowers distortion and increases headroom dramatically. It is not a subtle difference. I cross at 100 Hz 48 dB/oct. It does require a leap of faith because the only way you can do slopes this steep without causing problems is digitally and many audiophiles are also digital phobic. I have been using subwoofers since 1978 and it was a love/hate relationship until digital signal processing arrived in the mid 1990s with TacT Audio. Conversions back and forth in 192/24 digital are sonically invisible. Once you are in numbers you can go almost anywhere you want without any added distortion. Once you have everything set up as you want it you convert back to analog as the very last step.

There are other advantages besides sub management. High resolution EQ is one. We locate sound by volume and phase. If a sound is louder in the left channel you hear it to the left. The problem is that not two identical speakers have exactly the same frequency response curve. Put them in different locations and their curves can be wildly different. I just measured a set of Magico S7s that were down 8 dB at 500 Hz in one channel! What do you think this is going to do to the image? With digital EQ you can adjust each speaker's response curve so that the channels are identical producing the best image. It is way more important for the curves to be identical than flat. 

@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...