Are the cables of the NDH 20 compatible with the NDH 30 and vice versa?

Yes, all cables of NDH 20 and NDH 30 are interchangeable and available as accessories. The 2.5 mm connector with bayonet lock is identical on both headphones.
The cloth covered NDH 30 cable uses internal symmetric wiring for minimized crosstalk, providing razor sharp stereo location. It can also be used with the NDH 20 to take advantage of the benefits mentioned above.

Is it true that the headphone amplifier has an influence on the sound quality of headphones?

Yes, that’s true. The headphone amplifier’s output impedance and crosstalk damping have an audible effect on the sound character and stereo imaging. For optimal performance we recommend an output impedance below 5 ohms and a crosstalk damping of 50 dB or more.

What is the difference between the (open) NDH 30 and the (closed) NDH 20?

Open headphones are still the optimum in natural sound reproduction. The bass range can be presented drier and more defined, the midrange reproduction can be implemented free from resonances, and in the case of the NDH 30 the high-frequency reproduction is optimized with a very special absorber for maximum linearity.

Why do you recommend the NDH 20 for mixing purposes?

Monitor headphones used by musicians during recording session are normally closed to minimize crosstalk to the microphones. Open headphones are often used for mixing and mastering. The NDH 20 is equipped with a very linear frequency response, although it is a closed headphone and suppresses external noise very well. This makes it possible to edit and mix finished recordings even in noisy environments. A detailed review can be downloaded here: NDH 20 review.