The Sennheiser HD 660S2 is presented as a new step in one of the most recognizable lines of open-back dynamic headphones. Rather than moving away from the established 600-series formula, it refines it: a familiar audiophile-oriented platform, a high-impedance driver system, open-back construction, replaceable connection options, and a renewed emphasis on bass extension. For listeners building a dedicated headphone setup, the attraction is not one isolated specification, but the way the design choices point toward long, focused listening sessions with a broad range of music.
A continuation of the 600-series idea
Sennheiser positions the HD 660S2 within the same family as the HD 600, HD 650, and HD 660S. That matters because this line has long been associated with open-back listening, natural tonal aims, and a form factor designed more for home or studio-style use than for portable isolation. The HD 660S2 keeps that overall identity while introducing changes intended to extend low-frequency performance and increase driver control.
The headphone remains an open-back dynamic model. This is a deliberate design choice: open-back headphones allow air to move more freely around the driver and the ear, which can contribute to a less enclosed wearing experience. The trade-off is that they are not intended for blocking outside noise or preventing sound leakage. For a prospective owner, that makes the HD 660S2 more appropriate for a quiet room, desk system, or dedicated listening space than for commuting or shared public environments.
Why the revised low-frequency system matters
The most prominent technical emphasis in the HD 660S2 is its extended sub-bass behavior. Sennheiser describes a set of engineering changes involving optimized suspension, transducer airflow, reduced air leakage between the front and rear of the driver, and a redesigned coil. According to the supplied specifications, these changes double the sound pressure in the lowest octaves. In practical ownership terms, the point is not simply “more bass,” but a design attempt to give very low notes more presence without abandoning the headphone’s stated focus on precision and balance.
One of the clearest details is the change in resonant frequency, lowered from 110 Hz to 70 Hz through increased suspension compliance. In an open-back headphone, bass output can fall below the driver’s resonant frequency, so lowering that point is a meaningful design target. The documented examples are the lowest note on a piano at 27.5 Hz and bass drum fundamentals in the 45–60 Hz region. These are musical areas where sub-bass extension can influence scale, weight, and the sense of foundation beneath instruments.
The company also identifies airflow control as part of the update. Reducing air leakage between the front and back of the driver is significant because such leakage can behave like an acoustic short circuit, reducing low-frequency acoustic pressure. For listeners who appreciate open-back headphones but sometimes find them lighter in the lowest bass, this is one of the HD 660S2’s defining technical propositions.
A 300-ohm design for dedicated headphone systems
The HD 660S2 uses a redesigned coil with a 300-ohm impedance, matching the impedance of the HD 600 and HD 650. This is not a casual specification. A 300-ohm headphone often makes the most sense in a system where the owner has, or plans to use, a capable headphone amplifier, DAC/headphone amplifier, integrated amplifier output, or audio interface with suitable drive. The supplied material connects the new coil with greater magnetic force, improved electrical damping, lighter weight, and cleaner impulse response.
For system matching, the takeaway is straightforward: the HD 660S2 is aimed at listeners who are likely to care about the source chain. Its impedance and included cable options suggest use with home audio equipment rather than a phone-only setup. That does not mean the headphone is complicated, but it does mean it is best understood as part of a deliberate listening system.
Balanced and unbalanced cables included
Connectivity is one of the HD 660S2’s practical strengths. It is supplied with two 1.8-meter cables: one unbalanced cable terminated in a 6.35 mm TRS plug and one balanced cable terminated in a 4.4 mm TRRRS plug. This covers two common scenarios in modern headphone listening. The 6.35 mm connection suits traditional headphone amplifiers, hi-fi components, and studio-style outputs. The 4.4 mm balanced connection suits many contemporary desktop and portable headphone amplifiers that provide balanced headphone outputs.
The inclusion of both cables reduces the need to immediately source an aftermarket lead when moving between equipment. It also supports system flexibility: an owner can use the HD 660S2 with a conventional amplifier today and a balanced-output device later. The 1.8-meter length is well suited to a desk or nearby component rack, long enough for seated use without becoming an excessively cumbersome cable for typical home listening.
Comfort choices for extended sessions
The HD 660S2’s design is also built around long-session comfort. The supplied details mention velour ear pads, an ergonomic headband, elastic lining, low overall weight, and open-back ventilation. Each of these elements addresses a different part of headphone usability. Velour pads are often chosen for their soft feel against the skin. The headband structure is intended to maintain acoustic positioning while avoiding unnecessary distraction. The open-back format gives the ears a less sealed environment than closed-back designs.
This comfort focus is especially relevant because the headphone is not positioned as a quick-use portable accessory. It is meant for engaged listening, where an owner may spend an album side, an evening, or a work session wearing the headphone. Comfort does not replace audio engineering, but it determines whether the listener can actually benefit from the headphone’s design over time.
Frequency range and sensitivity in context
The published specifications list a frequency response of 8 Hz to 41,500 Hz and a sound pressure level of 104 dB. Frequency response figures should not be treated as a complete description of how any headphone will be perceived, but they do show that the HD 660S2 is specified for extension well beyond the conventional audible band at both ends. More importantly, the accompanying engineering notes explain where Sennheiser concentrated its revisions: the lowest octaves, driver suspension, airflow, and coil behavior.
The sound pressure level figure provides another useful reference point, though real-world loudness will still depend on the amplifier, source output, and listening habits. Combined with 300-ohm impedance, the specification set reinforces the idea that the HD 660S2 is intended for proper headphone outputs rather than being optimized primarily for maximum volume from low-power portable devices.
Who the HD 660S2 is most suitable for
The HD 660S2 is best suited to listeners who want an open-back headphone for home, desktop, or dedicated listening and who value the 600-series approach but want documented improvements in sub-bass extension. It is also a logical fit for owners of headphone amplifiers with 6.35 mm or 4.4 mm outputs, especially those who appreciate having both cable types included from the start. The $600 price places it in a serious enthusiast category, where build, comfort, connectivity, and system matching all become part of the purchase decision.
It is less suitable for listeners who need isolation, minimal sound leakage, wireless convenience, or a compact travel headphone. The open-back construction is central to its design, but it also means the headphone belongs in quieter environments. Likewise, the 300-ohm impedance will be most attractive to owners prepared to use appropriate source equipment. For someone building a focused listening station, these are advantages; for a purely mobile user, they may be limitations.
Conclusion
The Sennheiser HD 660S2 stands out through a clear set of documented design priorities: open-back dynamic architecture, 300-ohm driver impedance, revised suspension and airflow for stronger sub-bass output, included balanced and unbalanced cables, and comfort features aimed at long listening sessions. Its strongest appeal is to music listeners who want a home-oriented audiophile headphone with 600-series heritage and improved low-frequency extension, and who are willing to pair it with suitable headphone equipment.
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