Looking for a high-end HP amp (DIY kit)

There are plenty to chose from in terms of designs.

A couple of points:
a) Iterate on a good base.
b) a good design kits such as TomChr's HPA or Noir allow you to use a compatible wall block DC PSU.
c) components can cost considerable amounts and the likes of HPA/Noir etc will deliver top notch performance without requiring boutique cost components through good design.
d) Once you have it working you can tinker to your heart's content (point a).
 
I'm on the looks for a DIY headphone amp with low distortion and high-end sound. No tubes.
Should preferably be a full kit and in the price range 200-300€.
Any suggestions?
Class AB push pull driven by OPA2604AP, very very low distortion, variable GAIN, small and powerful 😉

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