sundara sound

I have a pair of Sundara closed backs. They play well off the phone, but really shine with my inexpensive Schiit headphone amp.

The Hegel has 270 mWatt output that should be ok for the Sundaras.  It’s not abnormal to have to turn the volume knob up a bit to get sufficient volume.  Do the headphones get loud enough?  If not then there may be a problem somewhere, and they also may require some playing time to break in to sound their best so there’s also that.  Just some thoughts FWIW. 

 The Sundaras need 126 mW to reach 85dB SPL.  You should have enough power, though the Hegel will require a higher volume volume setting, I expect.  The Mc should handle the Sundaras fine. 

Wiggle the jack connections in the phones and at the amp to see if there's a bad connection. ..or try a different cable. 

I received a small USB DAC recently..very nice DAC, but they included a charge-only (2 wire) USB cable with the unit...useless..oops. 

Sounds like bad phones. I would return them.  

OMG, C'mon. Really, "break in" on used headphones? That's not even a remote possibility! Contact Hifiman for help, don't waste your time!