Question

I have a Presonus Studio 68c, how many ohms should my headphones be to get a better efficiency?

Answer 1

Highlighted in Red below: 

Headphone Outputs

• Maximum Output

  150 mW/Channel (56Ω load)

• Frequency Response

  20 Hz to 20 kHz (±0.2 dB, 56Ω Load, 48 kHz)

• Dynamic Range

  107 dB (A-Weighted, 56Ω Load)

• Total Harmonic Distortion

  0.002% (1 kHz, -1 dBFS, no Load)

• Impedance Working Range

  32Ω to 300Ω

From the product tech specs - https://www.presonus.com/en-US/interfaces/usb-audio-interfaces/studio-series/2777700405.html