Perhaps the DAC has some sort of safety volume limiting feature enabled.
I'm sure the 5V output rating for the DAC-2 is the sum of the balanced legs. I suspect the 2.9V sensitivity for the amp is per leg as well when using the balanced connections. (Kudos to W4S for the sensible sensitivity rating.) Still, I'd expect 2.6V/leg into that amp to drive those speakers way beyond their power handling capability.
I'm sure the 5V output rating for the DAC-2 is the sum of the balanced legs. I suspect the 2.9V sensitivity for the amp is per leg as well when using the balanced connections. (Kudos to W4S for the sensible sensitivity rating.) Still, I'd expect 2.6V/leg into that amp to drive those speakers way beyond their power handling capability.