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.

