Overview

The Sennheiser HD 660S2 is the successor to the HD 660S, released in 2023. It features a 38mm high-performance dynamic transducer combined with an ultra-lightweight aluminum voice coil, with impedance changed from 150Ω to 300Ω. The frequency response spans 8Hz to 41.5kHz, achieving THD below 0.04% at 1kHz. Weighing 260g, this product inherits the traditional acoustic design philosophy of Sennheiser’s 6xx series while aiming for extended low-frequency response and improved high-frequency precision.

Scientific Validity

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Measurement results demonstrate good performance. THD is excellent at below 0.04% at 1kHz, falling well below the problematic level (0.1% or higher) though not reaching the transparent level (0.01% or below) from the reference table. Frequency response spans 8Hz to 41.5kHz broadly, but roll-off below 20Hz is confirmed, preventing complete transparency. While specific S/N ratio values are not published, the 300Ω high-impedance design suggests good results when paired with appropriate amplification. Measured frequency response is linear from 50Hz to 1kHz, with minor roll-off in sub-bass regions and slope in upper midrange frequencies.

Technology Level

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Technical advancement from the HD 660S is confirmed. Changing impedance to 300Ω and adopting a lighter voice coil improves damping characteristics and control. Resonant frequency also decreased from 110Hz to 70Hz, achieving low-frequency extension through more flexible surround design. Response characteristics above 5kHz are also improved, with reduced peak-to-valley differences achieving smoother characteristics. 40Hz square wave measurements confirm appropriate damping, and 100μs impulse response shows minimal ringing with good damping characteristics. These represent technical achievements exceeding industry standards.

Cost-Performance

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Against the 600 USD price, the “Drop + Sennheiser HD 6XX” offers equivalent 300Ω impedance, 0.05% THD, and similar acoustic characteristics for 199 USD. Calculation: 199 USD ÷ 600 USD = 0.332. Rounded to 0.3. The HD 6XX is based on the HD 650 design and measurement data shows substantially identical performance to the HD 650. No significant differences are confirmed in frequency response, THD, or impulse response measurements, providing acoustically equivalent performance at a lower price. While the HD 660S2’s technical improvements are commendable, the practical sound quality improvement users receive is limited, and performance differences do not justify the price gap.

Reliability & Support

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Sennheiser provides industry-leading reliability and support systems. While the 2-year warranty period is standard, repair systems and parts supply systems are extremely comprehensive. The 6xx series has been manufactured over extended periods, demonstrating parts supply continuity. Cables are detachable and replaceable, and consumables like pads are easily obtainable. Customer support response quality is high, with accurate responses to technical inquiries. Failure rates are below industry average, and stable performance maintenance during long-term use can be expected.

Rationality of Design Philosophy

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Improvements from the HD 660S are based on scientific evidence, confirming rational design philosophy. The 300Ω impedance change contributes to improved damping, and the lightweight voice coil leads to improved responsiveness. Decreased resonant frequency and improved surround flexibility have physical basis for low-frequency extension, and improvements in characteristics above 5kHz are supported by measurements. However, the practical impact of these improvements on human hearing is limited, struggling with differentiation from existing equivalent products like the “Drop + Sennheiser HD 6XX”. While the raison d’être as dedicated audio equipment is maintained, there is room for improvement from a cost-effectiveness perspective.

Advice

The HD 660S2 is technically excellent, but we strongly recommend comparing with the “Drop + Sennheiser HD 6XX” before purchase. The sound quality difference between the two is extremely subtle auditorily, and the HD 6XX’s 199 USD price provides overwhelming cost-performance. With a 600 USD budget, considering a combination of HD 6XX with high-quality DAC/amplifier can be expected to provide greater sound quality improvement. The HD 660S2 should only be chosen when one finds value in the latest technical improvements and can accept the price difference. For those already owning HD 650 or HD 6XX, upgrade necessity is judged to be low. For first-time purchases, we definitely recommend starting with the HD 6XX.

(2025.7.21)