Easy to drive, outstandingly natural sound from 40-50 Hz up.....AN-E, O/96, others?

I agree with your thoughts on the AN Duke.  Another option might be to use a high-pass filter like Vandersteen uses, and then roll the subs up to match the rolled off frequency and output of the main AN speakers.  Of course you may accomplish the same thing more simply by moving them out from the wall but the high pass would (theoretically) reduce the work required by the main amplifier.

BTW, I would like to hear your swarm system some day.  Adding a second Aerial sub was a huge benefit to the sound of my system, in particular by better controlling bass in my room. 

Duke,

I agree with what you say: "a delicious richness about them that really stood out from the crowd. One could easily get totally lost in the music listening to them. My impression was that they’d be totally non-fatiguing to listen to all day long."

The reason I’m interested in making a change is not because adding subs threw off the balance, it’s because I find the speakers to be a bit "sleepy." They simply don’t do anything wrong but they don’t generally grab me. My mind can easily wander when listening.
The reason I said I may never replace them is because I can listen for hours on end with absolutely no fatigue.
I’ve tried them with many different amps of different strengths and in an almost infinite number of positions within my room but the speakers simply are what they are. They sound best in the corners, set up exactly how Audio Note (and I assume Peter Snell) say to set them up. Driving them in this position with 2, 4, 8, 16, 20 or 250 watts doesn’t bring about the difference that I’m looking for.

But, in the end, I will not trade long-term listening enjoyment for excitement. 

mitch2,

I actually do use a high-pass filter between the preamp and the main amp. The main speakers are now down 3dB at 60Hz (9dB at 30Hz etc.). The device is passive and uses a small Audio Note UK capacitor.  This takes a lot of strain off the main amplifier. 60Hz and below is largely covered by the subwoofer system.

Fritz Carerra.

@exlibris 
Where did you find a good-sounding high pass filter made with high quality components, or did you make one yourself.
I have looked at Vandersteen's M7-HP, which is made for their Model 7 series speakers and appears to be well-constructed, but it costs $2K/pair and rolls off from 100 Hz, which is higher than I want.
https://www.vandersteen.com/categories/crossovers