There have been pages of debate about AK's EQ implementation in a number of the other product threads including the SE100 thread. Some of it got pretty nasty with one Head-Fier personally attacking and threatening me about it, so I'm not looking to get into another debate about EQ. Here is our official statement on EQ implementation on AK devices:
Astell&Kern uses a passive EQ as opposed to active EQ used by other manufacturers. Active EQ does require an amplification after the audio signal processing. This can be done in one of two ways - by adjusting the output levels, but it would limit the audio output power to less than what the device can actually deliver at its max. The other option is to put an additional amp in after the main Op Amp, and that is very hard to do in such a small space within a DAP. At the same time, the amplification of Active EQ may add greater possibilities of coloration or distortion, which we want to avoid at all cost. That’s why we chose a passive EQ so that users can make proper tonal adjustment with the least deviation from the original sound.
A&K’s incremental EQ settings in A&K are very, very subtle (0.1dB +/-) because we expect our listeners are only looking for finer adjustment. You will not find a 10 db+ adjustment on any of our players. If you are looking for a drastic EQ change, then Astell&Kern DAPs are not for you.
Astell&Kern has always had the philosophy of bringing the most accurate reproduction of sound like it was recorded in the studio, without alteration. That does not mean the music will sound better with Astell&Kern players. Instead, all of our devices will reveal everything from the digital music files no matter if the performances are beautiful or disappointing, we will not color the sound to make it "sound better." Astell&Kern does not believe in digitally altering the sound which is also why we do not provide DSP or digital filters on our players. Fans of Astell&Kern purchase our products for our unique sound signature which is tuned on the hardware level. We also would never alter that sound signature via a software update. Astell&Kern has never meant to deceive anyone and the passive EQ has been discussed many times over the last 6+ years on the AK Head-Fi forums, on social media, in previous reviews and when we meet people at shows. If you are looking for a DAP that you can digitally alter the sound, an Astell&Kern DAP may not be the correct fit for you.
Here’s a good discussion on Passive vs Active EQ -
https://www.soundonsound.com/sound-advice/q-what-difference-between-passive-and-active-eqs