I recall the original Alexx was quite a difficult load...from STereophile below:
Sidebar 3: Measurements Because this speaker's size and 452-lb weight made shipping one of them to me for measurement impossible, I drove my test gear to Michael Fremer's place. I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Wilson Audio Alexx's frequency...
www.stereophile.com
"...Although Wilson's specification is correct, in that the minimum magnitude (solid trace) is
1.5 ohms just below 3kHz—I measured 1.44 ohms at 2.9kHz—
the impedance remains at or below 3 ohms from 55Hz to 6kHz, and the
electrical phase angle (dotted trace) exacerbated the drive difficulty in some regions of the audioband. The
combination of 3 ohms magnitude and a phase angle of –44° between 56 and 59Hz will stress amplifiers, as will the combination of 2.2–2.6 ohms and a phase angle of +40° between 4 and 5kHz. Alexx owners need to match the speakers with amplifiers unfazed by very low impedances."
I think the new Alexx V may be less so but still appears to be 2 ohms at 250hz...also from Stereophile measurements:
Sidebar 3: Measurements Because of the Alexx V's size and bulk, I measured its performance in Jim Austin's listening room. I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Wilson Audio Alexx V's frequency response in the farfield, an Earthworks QTC-40 microphone...
www.stereophile.com
"...My impedance measurement is shown in fig.1. The impedance magnitude (solid trace) remains between 2 and 4 ohms for almost the entire audioband, with a minimum value of 1.9 ohms between 242Hz and 270Hz. The electrical phase angle (dotted trace) is low from the upper bass to the low treble, but there is a combination of 3 ohms magnitude and a phase angle of –39° at 53Hz that exacerbate the drive difficulty.
Using a spreadsheet prepared by Jim Austin, I calculated the effective resistance across the audioband (EPDR, footnote 1) that results from the combination of magnitude and phase angle. The EPDR drops below 2 ohms over most of the midrange and mid-treble regions, with minimum values of 1 ohm between 50Hz and 64Hz, 1.15 ohms between 197Hz and 203Hz, and 1.05 ohms between 3kHz and 3.16kHz. Owners of the Alexx V need to match the speakers with amplifiers that are unfazed by very low impedances."
Based on my own limited experience...seems like a job for Gyphon as a few have noted. It doubles all the way down to 0.5ohms and doubles again all the way down to 0.5ohms in peak. So for Mephisto monos, that is something like 6400 watts peak into a 0.5ohm load.