The Zähl HM1 might just be the very best headphone amplifier I’ve come across. It’s incredible in so many ways, but not easy to get ahold of one.
If you’d like to see full measurements they are posted here: https://headphones.com/blogs/reviews/zahl-hm1-review-measurements
Product Page: https://headphones.com/products/zahl-hm1-reference-desktop-headphone-amplifier
The home page link for this review reads
“The EF400 is an interesting product unfortunately let down by a few design flaws and oversights. Here’s my full review”
you may want to change that