I bought the first mainstream ultra-measurements headphone amp about 5 years ago: the THX AAA 789. While okay in controlling the issues of low-end headphones and not bad with IEMs, that lean and dry amp now sits in a box unused.
Per the linked thread, I recommend Class A solid state amps. The simple Class A design sounds pure, fuller, and just better to my ears. I personally own the Class A $600 RebelAudio Rebel Amp. However, in daily life I use a nice Decware tube amp most of the time.
If you were US based I’d suggest Schiit products as often warmer and richer. In the EU look at the iFi product line, such as the $200 ZenDAC. Their Burr-Brown chips are a lot easier on my ears than ESS chips.
I’m calling out @SenyorC and @Lothar_Wolf for EU-specific ideas in this price class.