sundara sound

They should play fine with your amp headphone jacks. Specs look to be 37 ohm and 94 dB for the Sundara.  I recently purchased a pair of Hifiman Ananda Nanos (14 ohm and 94dB)  and they play (surprisingly) fine directly plugged into a Samsung tablet, or a dedicated headphone amp (1.25 wats at 34 ohms) or the headphone jack of my MA6100 McIntosh integrated. 

You may have a bad cable?  Amazon has plenty of aftermarket, inexpensive cables you can pick up as a test cable. 

I'd contact Hifiman and see what they say.  The open box stuff has a std warranty I think. If they're faulty, they'll cover it. 

The Sundara is a really nice headphone, especially for the $$. Don't know why they aren't showing their wares on your set up but if you have a friend or relative with a dedicated headphone amp, plug them in and see what you think. Enjoy the music

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.