Unfortunately, in Greek, the term "neutral" or "flat" means indifferent, neither cold nor hot! However, in sound, it means exactly the opposite. Acoustically, headphones like the HD25 or HD600, which are considered highly neutral or flat, are definitely not indifferent.
In a few words, the HD25 has a crystal-clear sound, with all the details that even much more expensive headphones can provide. It is not a coincidence that even foreign reviewers on YouTube mention that the detail and clarity of the low frequencies can be compared to very expensive headphones like the LCD2. And yes, even though they are neutral, they have just the right amount of bass, punchy and detailed. And because they can produce 120db without distortion, they can handle as much EQ as one wants, turning them into bassheads, hypothetically. The mid frequencies are rich, providing warm and sweet male and female voices, with all their details, and the high frequencies deliver maximum clarity without ever being harsh.
As for the build quality, actions speak louder than words, as there are plenty of internationally renowned DJs who have been using them for 10 or 15 years and say that the headphones are like the first day they bought them. A look at consumer reviews on international online markets generally confirms the above and even more.
One of the best headphone purchases!
(The above is written based on experience with Sennheiser HD600, Audio Technica M50x, Sennheiser 380pro, Sony MDR 1a, V Moda LP2, Marshall Major II, Beyer Dynamic DT770pro, Sennheiser PXC550, Xiaomi Headphones, and IEMs Tin T2, Tin P1, Senfer Pro, Moni One, Fiio FH1, and Fiio FH5.)