Sennheiser released its HD 660S open-back headphones — which were the successor to the company's 14-year-old, beloved HD 650 — back in late 2017, and they almost immediately became a hit with budding audiophiles. They looked almost identical to their predecessors, but the HD 660S they boasted improved internals so that they sounded even better.
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Fast forward to 2023, and Sennheiser has basically done the exact same thing with the newest addition to the company's HD 600 series: the Sennheiser HD 660S2.
The most notable upgrade with the Sennheiser HD 660S2 is improved bass over the now antiquated HD 660S. Sennheiser updated the HD 660S2 with revised and smaller transducers (42mm vs the new 38mm) that offer improved airflow and have a more refined voice coil; this drops the resonant frequency from 110 Hz (original HD 660S) down to 70Hz. According to the press release, this helps the the HD 660S2 deliver "deeper, clearer bass without sacrificing the expansive detail the series is known for."
Another key difference between the HD 660S and the new HD 660S2 is with impedance. One of the big things when the HD 660S were released five years ago was that Sennheiser lowered the impedance — from 300 ohms to 150 ohms — so that they were easier to drive. Well, Sennheiser has reverted back to the old impedance of 300 ohms with the HD 660S2, which is the same as the company's older HD 600 and HD 650 headphones, so you'll want a external amp to drive to them.
As previously mentioned, the design of the Sennheiser HD 660S2 is pretty much identical to the HD 660S. They have the same plush earpads and padded headband with folding hinges. The only aesthetic difference is the bronze accents (instead of silver) on the new models.
And finally, there's the price — it's gone up. The Sennheiser HD 660S2 cost $599, which is $100 more than the HD 660S when they were released. You can preorder the HD 660S2 now, and they'll be available on February 21.