I bought these headphones at the beginning of the year. I chose this specific brand and model after doing a lot of research (watching YouTube reviews and checking the specs) and reading reviews on skroutz, wanting a much more advanced sound experience. I think the reviews are somewhat exaggerated, but this particular model definitely has very good features.

+ Pros
Truly clear sound with distinct instruments, even at maximum volume
They are not advertised as sound cancelling headphones, but they still pleasantly isolate the surrounding noise to some extent

- Cons
I heard/read a lot about how soft and lightweight they are. They are not heavy headphones, but after half an hour or an hour of use, I find myself needing to take them off my head for a few seconds and then continue wearing them. Also, the earpads are quite "warm" for my preferences.
Due to the perforated outer casing, if someone is close to you when you listen at above medium volume, they can hear it normally as well. So if you want to listen to music a bit louder and the person next to you wants to read or sleep, it can become a bit annoying.

Overall, it's a very good purchase. I won't even get into the financial aspect as it is relative for each person. For me, they are worth the money.

I will say that these headphones are the best for those who love music. They have incredible imaging soundstage and, due to their open-back design, they have an airiness that you can't find in closed-back headphones. They have one of the best frequency responses and, for open-back headphones, warm natural bass. Additionally, if someone wants more bass, they can easily use an EQ.

They are extremely lightweight for audiophile headphones and it is well-known that they are of the highest quality. A simple search on the internet will show how durable the HD 6 series is, lasting for decades. Now, if you are careless and throw them around like they are portable headphones, it is logical that they will break. Silk underwear also requires skilled handling...

Certainly, these headphones are not suitable for young gamers or YouTubers who play games like Fortnite in front of their PCs and throw their headphones around in frustration. There are affordable solutions for them, gaming headphones from Razer, Logitech, Sennheiser, etc., with bass that will make their heads shake.

These specific headphones are for audiophiles who understand sound, can discern what they are hearing, and of course, have the equipment.

Amazing headphones, I went and listened to several others up close but these headphones were by far the best. They have such a warm and analog sound and all of this with incredible clarity.

The build quality is well-known, the 6 series is the most durable over time and the use of high-quality plastic is intentional to keep them lightweight.

They are also open back with non-planar drivers, there is no better bass...there are others, but they are semi-open like the dt990.

The earpads are among the best in the world, made of velvet and leather on the inside, so good that even big companies like Dekoni couldn't make anything better.

It is well-known in the audio community for many years how durable they are and how comfortable they are on the ear, as well as the fact that the genuine earpads are the best.

Of course, it is understandable that they may not appeal to people who are used to closed-back headphones like Razer.

If you are an audiophile and appreciate sound quality, pair them with an amplifier and they will be perfect.

If you are not into sound, get a cheap one for 100 euros, they might sound better to you.

Owner of the HD5 and HD6 series, as well as the Drop 58X, 6XX, and other headphones. I hope to help those interested in purchasing.

A) Build quality: Here, you won't find expensive materials like leather, wood, aluminum, carbon fiber, like the Focal Clear, B&W, B&O, Audeze, etc. This may initially surprise you, but it's logical because the construction is entirely plastic with metal grilles. The reason is simple: the weight needs to be light and as compact as possible.
B) The driver is the same as the HD700 with the blue coil. The earpads are not made of memory foam because there is better breathability in the foam used in the originals. They also have velvet on the outside and leather on the bottom, which should never be changed. I have tried the Dekoni and Yaxi pads, but they all alter the sound to some extent, which I won't go into detail about. Genuine pads need to be changed once a year. The drivers have no flaws; after all, we don't want a huge soundstage here.
C) The sound is classic for the HD6 series. In other words, it has the best midrange and vocals available up to €1,500, and this is a fact. There is no low end due to the technology and open-back design, which is also logical. The imaging and separation are on the level of Stax, and those who know understand how amazing it is. You can hear the strings vibrating even when the drums are playing two meters away. You can hear the hoarseness in the voice and the air with incredible detail. Additionally, they are incredibly fast, on par with planar headphones, even when multiple instruments are playing simultaneously, none overpowering the other.

Now let's compare them to other headphones that I have listened to for several hours.
HD660s vs HD650: The HD650 remains the king in terms of smoothness and the veiled sound that makes poorly recorded tracks sound sweet and without harshness. However, the HD660s are much more analytical, clear, and faster, with superior imaging and separation. The trade-off is that they can sound a bit harsh and cold with poorly recorded tracks. So, it depends on what you prefer.
Hifiman Sundara vs HD660s: The Sundara is slightly more analytical and reminiscent of the Beyerdynamic sound. It is not as warm as the HD660s and doesn't have the same vocal performance. The bass goes deeper, but due to its coldness, it lacks sufficient mid-bass. So, in practice, if a track lacks low end, the HD660s will sound bassier. The HD660s are lighter and more comfortable overall. The reason for choosing the Sundara is slightly better resolution, metallic sound, and slightly better low end. They also have a larger soundstage than the HD660s, but the HD660s have significantly better imaging direction.
Beyerdynamic DT 1990 vs HD660s: From the beginning, you can tell that they are completely different. The DT 1990 goes deeper, has very bright highs, and a larger soundstage. Those who enjoy an intense and exciting sound will prefer the DT 1990. However, those who can't tolerate a cold sound and want high resolution without piercing their ears will find the HD660s better. Additionally, after many hours of listening at high volumes, the HD660s do not cause fatigue.

Personally, I believe that at this moment, someone who loves music should have two headphones.
Sennheiser HD660s and Focal Clear. Everything else is mediocre. These two are perfect no matter what you play through them.
And yes, the HD660s benefit from a better amplifier, 100%. Just like the HD650, the difference is not as noticeable as with the HD650 because the HD660s are already at a high level.

It is characterized as the successor to the HD650, although personally I would disagree. The HD650 that I listened to some time ago seemed quite colored with slow decay bass, although the HD650 transforms when paired with tube amps. Let's move on to the HD 660 S, for which I use the chord mojo as a source.

The bass is of good quality for its category, fast and tight, but it doesn't go very deep. It is not seismic bass that you will feel in your heart, so bassheads better look towards planars like the LCD2Classic. The source, of course, plays a role, but for open-back headphones, it has good and audiophile bass, and it is not bloated in any way. Bass 7/10

The mids are classic Sennheiser, and if you belong to this club, you will love them. Drums, violins, guitars sound with rich overtones, vocals are warm, intimate, engaging, and crystal clear. What impressed me is the timing, how tight everything sounds, a toe-tapping situation, especially with electronica/trance, but the star is the timbre. All instruments are reproduced in a completely natural and dynamic way, for example, the way a guitar string vibrates and the harmonic tone and the air that surrounds the flute. The mids are detailed (not at the level of the DT1990 PRO, which, although better in microdetail, has a drier and less musical tone), clear, rich, and there is no famous veil of the HD600/650 if the source is not up to par. Mids 8.5/10

The HD660S has more treble than the HD650, which is quite warm tonally with subdued highs, lacking energy in the high frequencies. The HD660S has more treble extension, plenty of air, it is more lively/exciting, but the sound is not sharp, piercing to the ears, nor completely smooth/dark like the HD650. The high frequencies are present, they do not play second fiddle to the mids, although there is a slight grain in the sound. They require a good DAC/amp and FLAC files, not below 320kbps MP3, otherwise, in bright low bitrate material, the sound may have some harshness and become fatiguing. Treble 8/10.

The soundstage is technically very good but not huge. That is the realm of 1000+ euro cans, like the HD800S. The width is not very wide but not claustrophobic either. You feel like you are in a medium-sized room, definitely not a concert hall, but the placement of the instruments is precise, and there is enough air between them. They do not sound condensed and mashed together. The depth is also very good, the sound is full and not flat, the 3D sensation is present, making classical music enjoyable even if the width is not overly expansive. Soundstage 8/10.

The HD660S excel in being technically clean, detailed, while also being musical (not for studio monitoring), and they perform every genre of music at a good level, but not perfect. They are definitely not for bassheads, after all, they are open-back (the music is heard in the surroundings, which is annoying for those around, but also external sounds enter). They are also not for those who want extreme energy. The treble is present, and you can hear the rich tone, for example, in violins, but the headphones are neutral, and all frequencies are in harmony. If the price goes down to 400 euros, I highly recommend them, but the source must be at a similar level. Forget about using them with mobile phones, even with an LG V20, they don't reach their maximum potential, especially in terms of dynamics, soundstage depth, and detail.
Something like audiolab m-dac plus for desktop solution, the Sony Wm1A/ZX300 in balanced for portable is a good choice unless money is not an issue, then Benchmark DAC3 HGC or Chord Hugo 2 if music is a way of life.