Are the HD9 good for mixing, or only for tracking?

They are designed for both. The closed-back isolation suits tracking, and the 10 Hz to 26 kHz response with accurate bass is tuned for mixing. Many engineers keep a closed pair like these alongside open-back headphones for checking low end.

Do the HD9 need an amplifier?

No. At 40 ohms they are easy to drive from a laptop, phone or audio interface. The 900 mW per channel rating means they can also take loud levels from a headphone amp without distorting.

Can I use them for recording vocals without bleed?

Yes. The closed-back cups reduce bleed into a microphone, which is why they are common for tracking. Isolation is good but not total, so very loud cue levels can still leak.

Is the cable detachable?

No. The ten-foot straight cable is fixed and drops from one side. That keeps it out of the way during sessions but means you cannot swap it for a shorter or balanced cable.

How do they compare with open-back studio headphones?

Closed-back models like the HD9 isolate better and leak less, which helps in tracking and shared rooms. Open-back rivals generally sound wider and more open, so the HD9 trades some soundstage for isolation.

What comes in the box?

The headphones with their attached ten-foot cable. No case, adapter or spare pads are included, so plan on adding a 3.5 mm to 1/4-inch adapter if your interface needs one.