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

Verdict: Both headphones offer strong performance, and the right choice depends on your priorities such as battery life, comfort, and noise cancellation.