Sennheiser HD 660S2

Pros

• Improved bass and sub-bass extension compared to the HD 600 and HD 650
• Rich, 'syrupy' midrange that excels with intimate female vocals
• Excellent stereo imaging and natural instrument decay
• High comfort level suitable for long listening sessions; lightweight build
• Pairs exceptionally well with tube amplifiers due to its high impedance

Cons

• High MSRP leads to frequent 'overpriced' labels compared to the HD 6XX
• Narrow soundstage that retains the traditional '3-blob' effect
• Recessed upper-mids (2-3k range) can make the tuning feel 'off' or 'weird' to some
• Requires dedicated amplification to perform optimally

Caveats

• Opinions are polarized on whether it is a meaningful upgrade or just a 'mildly different' version of older models
• The tuning is more colored and less strictly neutral than the legendary HD 600
• While bass is improved for the series, it may still lack the 'slam' found in planar headphones

Best for

• Vocal-centric music and listeners who prefer a 'rich' or 'refined' sound
• Daily driver use for those who find the HD 600 too lean in the bass
• Tube amp owners looking for a classic high-impedance pairing

Avoid if

• You are looking for a wide, expansive soundstage (consider Hifiman alternatives)
• You already own an HD 6XX and want a massive technical leap for the money
• You prefer a strictly neutral, reference-grade frequency response