With my recent listening station revamp, I edited my everyday setup down to the most interesting stuff:
WiiM Pro Plus → Bifrost 2/64 → Lokius
(A) XLR3 → Decware Zen Taboo Mk 4
(B) RCA → Bottlehead Crack (ボトルヘッド・クラック)
Both amps sound very good, but I’m feeding a $300 Crack kit with an $800 DAC. I’ve done quite a few back-to-back comparisons using the HD 800 S on the Decware and Beyer DT 880 600 ohm on the Crack. The timbre is not much different and they go head-to-head in the midrange, but the Decware/HD 800 S is a lot more technical. Deeper bass, higher highs, better staging, less cloudy, and purer treble.
The point of diminishing returns is real, and the headphone amp performance curve starts to bend around the price of the (cheap) Darkvoice 336 and Crack. I’ve not heard any Eufonika, and only demoed Feliks at audio shows. As such, I hesitate to make firm claims. Feliks products range from modest <$1,000 to $10K, so you have to specify.
“In the middle” seems right.