Sennheiser HD 660S2 and Beyerdynamic DT 1990 PRO MKII are well-known options in the wireless headphone segment, offering a balance of audio quality, battery performance, and smart features. Both models support modern Bluetooth connectivity, ensuring stable wireless performance with compatible devices. There is a noticeable difference in weight (260 Gm vs 376 GM), which may affect comfort during extended use. The frequency response differs (8 Hz -41500 Hz vs 5Hz - 40KHz), which can influence how well the headphones reproduce lows, mids, and highs. The driver size differs between the two (38 mm vs 45 mm), which can influence sound depth, bass response, and overall audio clarity. Sennheiser HD 660S2 and Beyerdynamic DT 1990 PRO MKII offer different noise cancellation capabilities (AMBIENT vs NA), which can impact how effectively they block external noise. Impedance levels vary (300 ohm vs 30 ohm), which can affect compatibility and audio output when used with different devices. Both headphones offer solid battery performance, making them suitable for long listening sessions and travel. From a usability standpoint, both models provide a reliable and premium headphone experience.

Verdict: Both models are competitive, and your decision should be based on your usage needs and preferred features.