Hi. I've followed this thread for quite some time. I may have contributed to it earlier, but I have mentioned here or elsewhere how pleased I've been with a single Benchmark AHB2 driving my pair of Montis ESLs in a sizeable living room.
I'd like to add some additional remarks concerning what, IMHO, ML ESLs need, and perhaps what they don't need or can't use.
I'd like to offer a pair of useful articles. The first is a multi-page review of the ML Montis speaker by Stereophile's Robert Deutsch with measurements by John Atkinson:
One of my formative audiophile experiences was the first time I heard electrostatic speakers. I walked into an audio store and heard music played by a live jazz combo. But where were the musicians? I saw none, though I did notice a couple of room-divider panels in the part of the store where the...
Note particularly the Specifications page and the Measurements page.
First, it's important to remember that, for most ML ESLs, the bass is handled by its own self-powered woofer amp. Over the bass frequency range (from 20 Hz to, say, 200-300 Hz), the ML ESL presents to the outside amplifier a nearly ideal flat resistive load that is a pussy cat for the outside amp to drive. The real delivery power comes from the ML ESL's own built-in bass driver amp.
On the Specifications page, the indication is the ML Montis has a sensitivity of 91 dB.
Benchmark Media provides a useful whitepaper with calculators concerning System Performance Calculations:
Benchmark's Audio Application Notes for Audio Professionals and Hi Fi Enthusiasts. Digital Audio, MQA, Analog Audio, Audio Myths, Audio Cable, Audio Measurement
With 100 watts of input and a speaker sensitivity of 91 dB, the Montis pumps out a sound pressure level of 111 dB SPL. At one meter distance, this is the level of an active jackhammer or power saw, certainly not levels that should be tolerated without ear protection.
Said another way, 100 watts is plenty to drive the Montis to levels beyond hearing safety.
Now look at the measurements. Notice that, at frequencies at or above 1 kHz, the Montis driving point impedance resembles a capacitor: the impedance drops inversely as the frequency rises, and it actually drops to as low as 0.5 ohm at the upper end of its range.
IMO, the best sort of amplifier for this type of load is one that is both tolerant of wide swings in impedance, AND that can deliver large amounts of clean current at high frequencies, which the AHB2 is designed to do. In fact, the AHB2's power output goes up beyond the nominal 100 watts per channel as the load impedance drops.
I'm not saying the AHB2 is the only amp that works well the the ML ESLs. I can tell you from my experience that it does work well, that its output power is more than adequate and its operational properties work with this ESL as a load. I don't think it's necessary to use more power than the speakers will ever be capable of processing and delivering. I've tried to explain why, but I know others may feel differently.
I've used other amps as well, notably a custom modified ADCOM GFA-555, a refurbished pair of McIntosh Mc60s (6550 tubes with a unique multifilar output transformer and wiring topology) and a Marantz 8B (EL34s). The tube amps had difficulty driving the ESLs at the upper frequencies (the speakers sounded sweet but dull), while the GFA-555 isn't nearly as clean and well behaved as the AHB2.
I do own an NAD M33 with a pair of Purifi Class D power amps. I have listened to the M33 a great deal with Dynaudio loudspeakers (remarkable results) but I have not tried it with the ML Montis.
It's worth taking a look at how the AHB2 has measured up. It's one of the most carefully measured amps out there. I'm not saying measurements are everything, but it is one of the cleanest, quietest, lowest distortion amplifiers in production.
There are good reasons for buying a specific amp or pair of amps -- pride in ownership, for one -- and far be it from me to be critical of any one else's favorite choice as being just that. In terms of what the ML ESLs actually need to work at their best, to achieve what really makes them sing, is not as elusive a matter for deliberation as it might have been in the distant past.
Many dealers offer extended try-out periods offering full money back (less shipping) if the performance proves unsatisfactory.
As always, these comments are just my two cents.