Sennheiser HD 660S2 Review: Exceptional Warmth and Musical Clarity

Experience the exceptional sound quality and warmth of the Sennheiser HD 660S2 headphones, blending classic design with modern performance.

Introduction to Sennheiser HD 660S2

The Sennheiser HD 660S2 is a dynamic, high-impedance headphone model priced around $430, serving as an updated version of the earlier HD 660S. This new iteration invites a fresh look to compare it not only with its predecessor but also with contemporary competitors such as iBasso SR3, OLLO S5X, SIVGA SV023, and HIFIMAN Edition XS.

Sennheiser, a German audio giant founded in 1945 by Fritz Sennheiser, has an impressive heritage in producing premium headphones and microphones. Today, about 2,800 employees worldwide continue this legacy. The brand is known for delivering outstanding audio quality, with their microphones being industry standards in television and film production. Although Sennheiser headphones are manufactured under the Sonova umbrella now, the original expert team remains behind the product design and quality.

Build Quality and Design Insights

The HD 660S2 comes in a straightforward package that includes a soft carrying pouch instead of a hard case. Importantly, it features a balanced 4.4mm cable with proprietary connectors familiar to those acquainted with the HD600 series, allowing seamless connection to devices like the iBasso DX320 MAX TI.

In terms of construction, this version surpasses the original HD600 and the prior HD 660S. The plastic used on the ear cups and headband is smoother and more pleasant to touch. Though initially tight on the head, comfort improves after a few weeks of use. The cable quality is excellent, and replacing it is unnecessary. The ear pads start off firm but soften over time, enhancing comfort further.

The lightweight headband (about 260 grams) features a soft cushion, offering excellent support without adding weight. The grille has a refined paint job, and the visible drivers provide a cool visual effect—one of the reviewer’s favorite design touches.

The ear cups offer side tilt adjustments to fit your head shape and size snugly. These headphones stay firmly in place, but given their power needs, they're best suited for home or studio use rather than outdoor activities.

The open-back design allows sound to flow freely, meaning the music is audible both inside and outside the cups, with no passive noise isolation. The protective foam inside guards the drivers against dust and debris, keeping the headphones pristine.

Technically, the headphones boast a sensitivity of 104 dB, a frequency response from 8 Hz to 41.5 kHz, and extremely low distortion below 0.04%. With a 300-ohm impedance, they require a powerful amplifier like the HIFIMAN EF400, Aune S9C PRO, or iBasso DX320 MAX TI to perform optimally. Manufactured in Ireland, the HD 660S2 offers a robust build free from creaks and rattles.

Sound Quality: A Warm Yet Detailed Listening Experience

The HD 660S2 was tested with various sources, including iBasso DX320 MAX TI, Hiby R6 III, EverSolo DAC-Z6, Astell&Kern ACRO CA1000T, Shanling M3 Ultra, and the tube amplifier Feliks Audio ECHO 2. Despite their precision and detail, these headphones also sound fantastic with less powerful sources due to their stable sound signature.

Their signature sound is naturally musical, rich, and warm, differing significantly from the original HD 660S. They approach audiophile perfection with exceptional resolution and a warm, inviting midrange that keeps listeners hooked for years.

The bass has been notably enhanced from the earlier model, offering impressive depth and clarity. This new bass response handles dubstep, electronic, and fast aggressive music effortlessly, without distortion even at high volumes (up to 110 dB). The bass isn't overemphasized but remains impactful and well-layered.

The bass guitar reproduction feels incredibly natural, vibrating with life when the source track contains substantial low-end. The upper and lower bass ranges blend seamlessly with the mid frequencies, suiting genres from rap to classical. This bass warmth adds a pleasant glow to all instruments.

Midrange frequencies receive a special focus; vocals—both male and female—sound expressive yet never harsh or fatiguing. The headphones balance detail and smoothness, making voices a joy to listen to.

The sound can be comfortably pushed to 120 dB, retaining clarity without fatigue. Textures are buttery smooth and harmonically rich, much less prone to distortion than other similarly priced headphones. This means you hear genuine music, not noise or harshness.

A standout feature is their ability to make you sing along. Regardless of your favorite music style, the midrange reproduction is so captivating that stopping listening is tough. Details and resolution are simply stunning.

The original HD 660S was detail-oriented but slightly dry and somewhat restrained. In contrast, the S2 offers fluidity, warmth, and a more pleasurable listening experience. It even passes challenging tracks like Eskimo Callboy’s "Cinema" with flying colors, maintaining control and clarity at all volumes.

High frequencies in the S2 are airy and extended but far less sharp or tiring than before, delivering a balanced and natural sound that complements the warmer tones. Bright source components like the FiiO K9 PRO or Aune S9C PRO enhance the already impressive soundstage, making it feel like sitting front-row at a live concert.

The HD 660S2 excels at live concert recordings and studio precision alike. Cymbals pop with natural texture, never sounding muted or metallic. This versatility makes them a favorite for diverse music tastes, from Rob Zombie to cinematic scores like the Barbie soundtrack.

Comparisons with Other Models

Sennheiser HD 660S2 vs. HIFIMAN Edition XS

The Edition XS features larger, softer ear pads but a heavier build. It’s easier to drive and offers a wider, more spacious soundstage. The HD 660S2 focuses on smoother, warmer mids, while Edition XS delivers sharper highs. Both have excellent instrument separation but differ in sound signature: HD 660S2 is smooth and musical; Edition XS is more neutral.

Sennheiser HD 660S2 vs. iBasso SR3

The SR3 is lighter with softer ear pads but heavier overall due to design. SR3 provides a neutral, bright sound with less bass, while HD 660S2 is warmer, richer, and more energetic. SR3 is transparent; HD 660S2 is romantic and smooth with a pronounced midrange.

Sennheiser HD 660S2 vs. Sennheiser HD 660S

The HD 660S2 improves on build quality with better plastics and finishes but keeps a similar design. Sonically, the S2 is warmer, with deeper bass, more musical midrange, and a slightly wider soundstage. The highs are airier and less fatiguing, making it a worthy upgrade.

Sennheiser HD 660S2 vs. OLLO S5X

The OLLO S5X uses wooden cups, giving it a heavier feel. It’s easier to drive than the HD 660S2 but has a denser, warmer, sometimes over-bassy sound. Sennheiser’s focus is on a more open and brighter presentation, especially in the mids and highs, making music feel more natural and organic.

Sennheiser HD 660S2 vs. SIVGA SV023

The SV023 is lighter and easier to drive, with wooden ear cups that add weight. SV023 sounds more natural and neutral with pronounced mids, while HD 660S2 leans towards warmth and fullness with slower bass and more musical mids. The HD 660S2 adds a magical touch to music that the SV023 keeps more transparent.

Value and Conclusion

Right from the start, the Sennheiser HD 660S2 impresses with an outstanding price-to-performance ratio. They hold their own against any competitor due to years of refined acoustics and design. These are essential headphones for collectors and casual listeners alike, built to last and provide enjoyment for many years.

The HD 660S2 earns a special place in the audiophile hall of fame for its superb musicality, value, and craftsmanship. It takes a great headphone and perfects it with warmth, smoothness, and deeper bass.

If you're after warm, natural sound with plush ear pads, lightweight comfort, and a balanced cable included out of the box, the HD 660S2 is a heartfelt recommendation for almost anyone, no matter their age or music preferences.

Technical Specifications

• Driver Type: Dynamic, open-back
• Driver Size: 38 mm
• Frequency Response: 8 Hz – 41,500 Hz
• Impedance: 300 Ohms
• Sensitivity: 104 dB (1 kHz, 1 Vrms)
• Total Harmonic Distortion (THD): <0.04% (1 kHz, 100 dB)
• Ear Cup Style: Over-ear
• Weight: 260 g
• Adapters: 6.3 mm to 3.5 mm
• Connectors: 6.3 mm plug, balanced 4.4 mm plug
• Cable Length: 1.8 m
• Made in: Ireland
• Magnetic Field: 4.5 mT

"The new HD 660S2 sounds smooth, fluid, and pleasant, with an open, separated sound that I can enjoy with any music." — Reviewer

Frequently Asked Questions

• What type of headphones is the Sennheiser HD 660S2?
  They are dynamic, open-back headphones with high impedance designed for audiophile listening.
• Are these headphones comfortable for long listening sessions?
  Yes, after a break-in period, the ear pads soften, and the lightweight design ensures comfort even during extended use.
• Do the HD 660S2 headphones require an amplifier?
  Yes, due to their 300-ohm impedance, a powerful headphone amplifier is recommended to get the best sound.
• How does the HD 660S2 compare to the original HD 660S?
  The HD 660S2 offers warmer sound, better bass depth, improved build quality, and a more open soundstage.
• Do these headphones isolate outside noise?
  No, they are open-back headphones and do not provide passive noise isolation; they let sound in and out freely.

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