The frequency response differs (10 Hz - 40,000Hz vs 8 Hz -41500 Hz), which can influence how well the headphones reproduce lows, mids, and highs. Both models support modern Bluetooth connectivity, ensuring stable wireless performance with compatible devices. There is a noticeable difference in weight (310 g vs 260 Gm), which may affect comfort during extended use. In general, both options are suitable for everyday use including music, calls, travel, and work-from-home scenarios. Impedance levels vary (32 Ohms vs 300 ohm), which can affect compatibility and audio output when used with different devices. Denon AH-MM400 and Sennheiser HD 660S2 are well-known options in the wireless headphone segment, offering a balance of audio quality, battery performance, and smart features. Both headphones offer solid battery performance, making them suitable for long listening sessions and travel. Denon AH-MM400 and Sennheiser HD 660S2 offer different noise cancellation capabilities (Yes vs AMBIENT), which can impact how effectively they block external noise. The driver size differs between the two (40 mm vs 38 mm), which can influence sound depth, bass response, and overall audio clarity.

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