So, lets look at the AHB2 versus some other amps for 1kHz and see what distortion looks like at very low levels.
Doing some rough estimates based on this white paper:
https://benchmarkmedia.com/blogs/application_notes/interpreting-thd-measurements-think-db-not-percent...
For simplicity, just pick the lowest measured THD under 1W from the Stereophile measurements below:
.0005% THD => -106dB (Benchmark AHB2, $3k)
.006% THD => -84dB (Mark Levinson No 536, $15k each)
.02% THD => -74dB (Emotiva XPA Gen 3, $1k)
Now, assume we want a playback level of 80dB, which means we should plan for 100dB with peak.
100dB + each of the calculated values above yields:
-6dB Benchmark (won't hear any distortion)
+16dB Mark Levinson
+26dB Emotiva
This kind of illustrates what I heard when I swapped my amps out. I think of it like this, your ear can hear different frequencies and different levels for those different frequencies at the same time. So if you have a background singer and a main piano track, you are hearing multiple frequencies at once (and some are at low levels) and so these ultra low wattages become very important for background detail. Another place this really came into play was hearing the echo and decay of the room in the recording, artists breathing, etc... These affects were all masked before with my previous amps (although they were decent amps for the price).
It is true that this is all environment dependent. If a room's noise floor is 40dB with nothing turned on it will be harder to hear this difference. I have my theater very well treated and use horn speakers too which focus to the main listening position. I can definitely hear someone beside me talk when I play something at normal level, so to me the distortion difference I saw between amps must be audible.
I have no idea of how the HPA4 will behave versus HPA2, but given that swapping all my amps out with one of the same topology, I am really hoping for some audible distortion improvement.
image:
https://www.stereophile.com/images/1115BAHB2fig04.jpg
Fig.4 Benchmark AHB2, sum of distortion harmonics (dB ref. fundamental)
vs 1kHz continuous output power into 8 ohms.
image:
https://www.stereophile.com/images/1115BAHB2fig05.jpg
Fig.5 Benchmark AHB2, sum of distortion harmonics (dB ref. fundamental)
vs 1kHz continuous output power into 4 ohms.
image:
https://www.stereophile.com/images/1115BAHB2fig06.jpg
Fig.6 Benchmark AHB2, bridged-mono mode, sum of distortion harmonics (dB ref. fundamental)
vs 1kHz continuous output power into 8 ohms.
Read more at
https://www.stereophile.com/content...er-amplifier-measurements#wA2SWl8iAhXKwekc.99
image:
https://www.stereophile.com/images/617ML536fig04.jpg
Fig.4 Mark Levinson No.536, distortion (%)
vs 1kHz continuous output power into 8 ohms.
image:
https://www.stereophile.com/images/617ML536fig05.jpg
Fig.5 Mark Levinson No.536, distortion (%)
vs 1kHz continuous output power into 4 ohms.
image:
https://www.stereophile.com/images/817Emotivafig04.jpg
Fig.4 Emotiva XPA Gen3, distortion (%)
vs 1kHz continuous output power into 8 ohms.
image:
https://www.stereophile.com/images/817Emotivafig05.jpg
Fig.5 Emotiva XPA Gen3, distortion (%)
vs 1kHz continuous output power into 4 ohms.
Read more at
https://www.stereophile.com/content...er-amplifier-measurements#C4U8dJhrEy1EWAex.99
Read more at
https://www.stereophile.com/content...er-amplifier-measurements#HA5O0PiPD8e4tGTt.99