Anthem claims that the M1 has a high conversion efficiency, which was borne out by the fact that although the units did get quite warm, the temperature seemed to remain pretty constant, largely irrespective of the power levels the amplifiers were asked to deliver.
Listening tests began by stacking the two units and placing them onto a flat granite slab, simply substituting them for the Naim NAP500 that is normally used in the system. The rest of the system in use at the time consisted of FM radio (Magnum Dynalab MD106T), vinyl (a hybrid Linn/Rega turntable with Soundsmith SG cartridge) and a Naim NDX/UnitiServe digital source, all feeding a Naim NAC552 preamp with DR power supply. Speakers were Bowers & Wilkins 800 Diamonds, chosen not only because of their fine sound quality but also their prodigious power handling capacity. (And yes, they did survive!)
First impressions were generally very positive, with an exceptionally low (effectively inaudible) noise floor, and a crisp, clean sound with notably explicit vocal delivery. This is partly because the bass region is not only very clean but also on the dry side, while the extreme top end of the audio band sounds a little rolled off and lacking in air and transparency.
That combination of these two observations probably explains the amplifier’s mild tendency to emphasise the midband and presence parts of the audio spectrum. Intriguingly, this has the dual effect of simultaneously emphasising and dulling speech, so that the end result projects a voice strongly, but without the full crispness that assists intelligibility. While this might deliver fine midband dynamic expression, it also somewhat reduces speech intelligibility at very low volume levels.
As I turned up the wick, the sound certainly got louder…and louder, until I feared for both the speakers and my ears. However, it also seemed to become progressively harsher as the volume was gradually increased, and I noticed that the top of the uppermost amplifier was vibrating quite significantly. Mindful of the fact that neither amplifier was precisely flat, I decided to use tripod supports – a Vertex AQ Kinabalu underneath the lower one, and (so as not to scratch its top with spikes) Synergistic MIGs between the two amps. Now securely tripod supported, vibration in the casework seemed much reduced, and so was most of the high level harshness.