Overall, both headphones are feature-rich and cater to users looking for a combination of sound quality, comfort, and wireless convenience. The driver size differs between the two (38 mm vs 53 mm), which can influence sound depth, bass response, and overall audio clarity. Both models support modern Bluetooth connectivity, ensuring stable wireless performance with compatible devices. Both headphones offer solid battery performance, making them suitable for long listening sessions and travel. Impedance levels vary (300 ohm vs 40 ohm), which can affect compatibility and audio output when used with different devices. If you're comparing Sennheiser HD 660S2 and Audio Technica ATH-AD2000X, both models offer a strong combination of sound performance, comfort, and modern wireless features. There is a noticeable difference in weight (260 Gm vs 265 GM), which may affect comfort during extended use. Sennheiser HD 660S2 and Audio Technica ATH-AD2000X offer different noise cancellation capabilities (AMBIENT vs Active Noise Cancellation), which can impact how effectively they block external noise. The frequency response differs (8 Hz -41500 Hz vs 5Hz - 45KHz), which can influence how well the headphones reproduce lows, mids, and highs.

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