Sennheiser HD 820

Pros

• Relatively easy to drive compared to other high-end headphones
• Provides excellent isolation for office or shared environments
• Real-world bass performance often sounds better than measurement rigs suggest
• Unique sound signature that serves as an interesting complement to more neutral headphones

Cons

• Highly polarizing stock tuning described by some as a 'mess'
• Difficult to achieve a consistent seal due to the headband design inherited from the open-back HD 800
• Noticeable dips in the lower midrange and sub-bass without software correction
• Questionable value proposition at full retail price compared to competitors

Caveats

• Almost universally recommended to be used with Parametric EQ (PEQ) to fix tuning flaws
• Sound quality is extremely sensitive to head shape and the resulting physical seal
• Measurement rig data may be misleading as in-ear microphones show a more favorable bass shelf

Best for

• Users who need the isolation of a closed-back but want an HD 800-style soundstage
• Enthusiasts comfortable with tinkering and applying EQ profiles
• High-end office setups where sound leakage must be minimized

Avoid if

• You want a 'safe' or standard neutral tuning out of the box
• You are unwilling to use software equalization
• You have had fit or seal issues with the Sennheiser HD 800 series headband