There are many good comments in this thread but there is also much erroneous information. In most cases, this erroneous information is based on the assumption that the AHB2 behaves like most power amplifiers. The statements made would be true of most power amplifiers, but do not apply to the AHB2.
The AHB2 does not act like a traditional power amplifier because it has a very unique design. It has tightly regulated power supply rails, a patented feed-forward error correction system, and an offline protection system that is not in the signal path.
Unlike virtually all other power amplifiers, the THD produced by the AHB2 does not increase under load. The 8-ohm and 4-ohm THD vs. Power curves are absolutely identical. Even when loaded with 2 ohms, the THD is virtually unchanged. To the best of my knowledge, no other power amplifier can do this, or even come close to doing this. And, no amplifier delivers lower THD or lower noise at any price. The AHB2 holds the record for lowest THD and lowest noise. It is also important to understand that this performance is not just achieved at one particular load impedance, test frequency, or operating level. It is achieved into any rated load at any power up to full output power, and any test frequency between 20 Hz and 20 kHz.
The red clip lights on the AHB2 are driven by a circuit that measures the THD. If it exceeds 1% the lights will turn on. The AHB2 delivers an astonishing 0.00011% THD, 1 kHz at full rated power into any rated load. The transition from 0.00011% to 1% is abrupt, but it occurs above the rated output power. This means that the AHB2 stays clean, even when you drive it almost to the rails. Ordinary amplifiers cannot do this. With most amplifiers, THD gradually increases with output level and with increasing current load. With the AHB2, THD is not a function of output current.
The impedance load ratings for the AHB2 are based upon long-term thermal limits (prolonged operation into the specified load impedance). This is not a limitation on the allowable minimum impedance of the loudspeaker. It is a rating for the nominal impedance of the loudspeaker. If the impedance drops to 1 or 2 ohms at certain frequencies or is presented with a difficult phase angle, this is not a problem for the AHB2. Feed-forward correction is stable into any load. Feedback is not stable into any load. The AHB2 uses a large amount of feed-forward correction and a minimal amount of feedback. This allows stability into virtually any load. Match your speakers to the AHB2 on the basis of nominal impedance not minimum impedance. The impedance minimums are not a concern.
