emaillists,
I enjoy the Enigmacoustics Sopranino with my Audiostatic 240 electrostatic main speaker. I use the low crossover setting at 8 kHz, which greatly increases the output into much lower freq, compared to the high 12 kHz setting. Even though the crossover slope is 12 dB/octave, there is quite significant output at midrange freq. So the benefits of the Enigma are not due to more HF extension, but to more emphasis of upper midrange overtones. That's why bass instruments have more clarity--more overtones are revealed, displaying the bite of the instrument.
The only caveat is the parallel connection to the main speaker, which will greatly reduce the net impedance of the combination of the main speaker + Enigma. Stereophile had informative measurements of the Enigma impedance at the 8, 10, 12 kHz settings. At 8 kHz, the impedance dips as low as 1.75 ohms. At 12 kHz, the impedance is about 4 ohms. If the amp has a weak power supply, the output will be much lower at very low impedances.
Time alignment of the Enigma with the main speaker is important. Align the driver in the throat of the Enigma with the tweeter throat of the main speaker. The front of the horn of the Enigma will be quite forward compared to the main speaker. There is a big improvement in clarity with proper time alignment.