I have the HD600 which is 1-2 dB less sensitive below 2 kHz (~2 dB less at 1 kHz) than the HD650.
Source: https://www.reddit.com/r/headphones/com … e/ere3b59/
The highest I have listened to my HD600s on the ADI-2 DAC was 5 dB below full output using techno and 7 dB below full output using metal and while that was very loud, I could not hear any distortion from the amp.
The ADI-2 DAC has enough output voltage to drive the HD650 to ~123 dB SPL according to Sennheiser's specs (103 dB SPL at 1 V RMS). However, I find Sennheiser's sensitivity specs questionable. The sensitivity of the HD660S is fair bit more than the 1 dB increase over the HD650 stated in the manual; it's about 5 dB at 1 kHz at 1 V RMS (~109 dB SPL vs. ~104 dB SPL, source: https://youtu.be/mbxdRYnab9o?t=328). However, the website now says it's 5 dB higher...
Further, the voltage in the first measurement is ~-14 dBu which would indicate a sensitivity of ~91 dB SPL at ~-14 dBu at 1kHz + 14dBu = ~105 dB SPL at 0 dBu at 1kHz or ~107 dB SPL at 1 V RMS (~2 dBu) at 1 kHz which at the 22 dBu the ADI-2 DAC can output would give ~127 dB SPL at 1 kHz. The 4 dB difference may be explained by differences in measurement technique and applied compensation curve.
Given all the available measurements, the sensitivity should be between 103 and 107 dB SPL at 1 kHz at 1 V RMS giving between 123 and 127 dB SPL at 1 kHz at the 22 dBu the ADI-2 can output. Due to the frequency response of the HD650, this should be about the same (think +-3dB) from 50 Hz to 10 kHz. Below that the bass drops off and above that the measurements are unreliable.
If you listen at reasonable levels, the output capabilities of the ADI-2 DAC should be more than enough.
EDIT: There are some listening impressions of the HD650 using the ADI-2 DAC at the bottom of this test: https://www.audiosciencereview.com/foru … -dac.2582/.
I did not do any comparison testing this time. May do more later. But did listen a fair bit using both my Sennheiser HD-650 and HiFiman HE400i. I was pleasantly surprised to be able to drive the former as loud as I wanted. The sound was excellent, with great resolution, absence of any noise, and lots of power.