Just picked one of these up after reading and looking at the graphs posted by OP. I liked the variation of the amp and it intrigued me so I listened to it at my local McIntosh dealer with two different sets of headphones. Atriums and LCD- 2 Fazer. Based on the way the amp is designed this is how you can get different sounds out of it and limitations based on how much input level you have.
Taps: If you go higher up you will get a darker sound stage the greater the difference between the headphone ohms and the output impedance. You are pushing the tubes with a wider voltage swing and 2nd order harmonics, 3rd etc will creep in. Which isn't necessarily a bad thing. For instance on the AKG K52 setting it at 100 ohms sounds much better to me being less bright.
In terms of the graphs listed, McIntosh’s gain control isn’t linear; it adjusts the ratio between the tube’s local feedback network and the driver’s input sensitivity. You notice the ratio of 2nd order harmonics at the 2 o'clock position actually goes down by percentage of the fundamental.
Beyond that point ≈ 2 o’clock, global feedback increases again or the stage saturates differently, so distortion spectra flatten a bit — you get higher power and SNR, but slightly less warmth. You get more volume and less harmonic distortion.
12'oclock least amount of harmonic distortion but you need A Powerful preamp to get decent volume.
1'oclock you get the most 2nd order harmonic and other harmonics based on the fundamental and get a more tube sound.
2'oclock you get a little less 2nd order harmonic and even less 3rd order and get a tad bit less tube sound.
I am using a Fosi K7 and to be frank there is not enough preamp omphh to listen to headphones and the loudness I want. But if I wanted a clean signal I just plug it into the K7. If I want the tube sound the 1 o'clock seems perfect for all my headphones and I get the harmonics.
It's a great sounding headphone amp and honestly due to the flexibility I think it will become a classic once the user understands how to use it. If I've made a mistake please let me know.
