Launched in 2023, the Sennheiser HD 660S2 is the latest entry in the HD600 series. It builds on the still-in-production HD 600, a true legend in both professional and audiophile circles since its release in 1997. Like all Sennheiser models, the HD 660S2 uses a dynamic driver.
The HEDD D1, released in late 2025, is the company’s first dynamic headphone. HEDD is already known for its AMT ribbon-driver models, the HEDDphone and HEDDphone TWO, and the brand’s core reputation comes from its active studio monitors for professional use.
The HD 660S2 typically sells for about 450 USD, notably less than the D1 at 799 USD.
SENNHEISER HD 660S2 SPECIFICATIONS
- Circumaural open-back design
- Dynamic driver with 38 mm diaphragm
- Impedance: 300 Ohm
- Sensitivityl: 104 dB/1V
- Frequency response: 8-41,500 Hz
- Distortion factor: <0.04% (1 kHz, 100 dB)
- Velour ear pads
- Cables: 1.8 m (6.3 mm stereo jack) + 1.8 m cable (4.4 mm balanced) + 6.3 mm to 3.5 mm stereo adapter and carrying pouch
- Weight: 260 g
Current price and availability:
- Amazon: HD 660S2
HEDD D1 SPECIFICATIONS
- Driver: Dynamic TPCD driver
- Impedance: 32 Ohms
- Sensitivity: 100 dB
- Cable: 2 m detachable
- Cable headphone connectors: dual 3.5 mm
- Cable termination: 3.5 mm jack with screw on adaptor
- Semi-circular detachable pads “lip style”
- Modular design with replaceable parts
- 5 year warranty
- Weight: 356 g (measured)
Price at launch: $799 (excl.tax), €699 (incl. VAT), and £599 (incl. VAT).
BUILD AND COMFORT
The HD 660S2 is, for all practical purposes, essentially identical to the rest of the HD600 series.
Both the D1 and the HD 660S2 have a utilitarian, understated design. They feature padded headbands and replaceable ear pads, though the attachment systems differ: the HD600 series uses a proprietary mechanism, while the D1 uses the common lip-and-groove design that supports a wider range of aftermarket pads. Even so, there is still a solid selection of third-party pads available for the HD600 line.
Both headphones offer a reasonable amount of ear cup swivel, but neither is foldable. They also use detachable cables that connect to both ear cups. The HD600 series relies on proprietary 2-pin connectors, while the D1 uses standard 3.5 mm jacks, making cable replacement and upgrading more convenient.
LISTENING COMPARISONS
Associated equipment:
- Amplifier: Topping A90
- DAC: RME ADI-2 DAC FS
Språk by Building Instruments
I like this track. It’s a nice blend of instrumental jazz and alternative pop. The two headphones sound quite similar in overall tonality, though the D1 is a bit tighter and clearer. The bass is especially more controlled, and overall instrument separation is noticeably better.
Love Your Grace by Vega Trails
Excellent double bass and gentle piano on a dreamy track. Both headphones perform well with a similar overall tonality, but the D1 is tighter and cleaner, most noticeably in the lower end.
Moon Relay by imi
The rhythmic, hard-hitting bass has stronger impact with the D1, which also offers greater space between instruments, even though the overall tonality is quite similar.
Rambling Man by Laura Marling
Beautiful delivery from the HD660S2, natural, smooth, and sweet sounding. The D1 is more different here. While the sweeter sounding vocals takes the spotlight with the HD660S2, the D1 is more “scattered” or “busy” sounding because all details are better articulated.
Maridalen by Maridalen
Double bass and two saxophones playing beautifully in tune. Both headphones sound excellent, but the D1 is tighter with better definition and articulation, while the HD 660S2 comes across a bit more diffuse.
Spontaneous Compositorius by Thomas Agergard
This is one of my favorite tracks for evaluating treble character. In my setup, the HD 660S2 has more zing in the upper regions, with a touch more sparkle. The D1 sounds darker yet still very natural and articulate, just more controlled and reserved. The double bass is very tactile and tight on the D1, while a bit softer on the HD 660S2. The saxophone sounds lovely on both, but more upfront and articulate on the D1.
Vicarious by Tool
When the riff hits, the HD 660S2 sounds smoother and more relaxed, drawing your focus toward the vocals. The D1, on the other hand, brings out more detail and provides better instrument separation.
Escape Route by Boris Blank
Again, the D1 is tighter, clearer, and more articulate, revealing “every detail” in the music. The HD 660S2 feels more musical, guiding your attention toward the main themes.
Young Vivaldi by Modo Antiquo
They are not vastly different, but the D1 feels more neutral and articulate, while the HD 660S2 offers a smoother, more fluid delivery.
Summer 3 Vivaldi Recomposed by Max Richter
Both the D1 and the HD 660S2 have excellent instrument timbre, delivering smooth and engaging sound with clear detail. The HD 660S2 is even smoother, while the D1 offers better articulation.
WRAPPING IT UP
Sound Signature
Both headphones are fairly neutral and designed for studio use, each achieving a natural, balanced sound. The HD 660S2 has slightly more sparkle, while the D1 features tighter, more prominent bass. The D1 is more articulate, the HD 660S2 is a bit smoother.
Treble
Neither headphone has overly bright or harsh treble, but the HD 660S2 offers a bit more sparkle. The D1 delivers plenty of detail and articulation while being slightly more restrained, ensuring a fatigue-free listening experience.
Midrange
The midrange has always been a strong point of the HD600 series, and the HD 660S2 continues this tradition. It is smooth and pleasing but slightly less articulate than the D1, which has a more upfront and detailed midrange.
Bass
The HD 660S2 improves on the bass extension and performance of its predecessors, but the D1 remains superior, offering tighter bass with better extension and linearity.
Soundstage and Imaging
The soundstage of both headphones is similarly open, but the D1 offers better detail, clearer instrument separation, and a darker background, making elements stand out more, while the HD 660S2 feels smoother overall.
Detail, Dynamics, and Timbre
They are not miles apart when it comes to detail tribal, but for the most part I find the D1 to offer more detail, along with better microdynamics and macrodynamics.
Timbre differs slightly, but both headphones have fairly neutral tuning. The HD 660S2 is smoother and can feel sweeter and more enjoyable on some tracks, while other tracks sound more natural and engaging with the D1.
CONCLUSION
Overall, these two headphones are quite similar, but with some differences. The HD 660S2 delivers a smoother, sweeter, and more “musical” sound, while the D1 is tighter, more articulate, and precise, offering better detail, bass control, and instrument separation.
Both are neutral and natural sounding, so preference comes down to taste. Technically, the D1 is superior in my setup, but the HD 660S2’s slightly less precise yet charming and smooth delivery has an appeal.
Thanks for reading. You can support us by purchasing anything using any of our affiliate links.
- Amazon: HD 660S2
Any purchase you make on Amazon or Linsoul with any of our affiliate links will give us a small provision at no cost to you.
We only get a provision for items that are not returned, so there’s no incentive for us to recommend something that’s not good.
Linsoul : Headphones, Earbuds, Wireless Earbuds, Desktop DAC/AMP, Portable DAC/AMP, Digital Audio Players,
Amazon: Headphones, IEMs, Headphone Amplifiers, Home Audio or Anything else.
.
If you enjoyed this article or other content on The Headphoneer, you might consider leaving a small donation to keep this website up and running. No donation is too small. Thanks for supporting us!
If you like our work please follow us on Instagram, Facebook and Twitter , it will help us grow. Sharing is caring 🙂
