Focal Clear Mg wins on raw dynamics and soundstage width with 40mm magnesium drivers producing 104 dB sensitivity (per Focal specs), outperforming the HD 660S2's 104 dB at 300 ohms on bass-heavy tracks. Sennheiser counters with lighter 260 g weight and smoother frequency response up to 41.5 kHz (per Sennheiser data), making it preferable for vocal accuracy. RTINGS and Audio Science Review data ...