Question #1: did the music audible through the speakers match the music being received by the DAC? Whether or not it was the same will affect the diagnostic.
A Ghost in the System? Music Thru Speakers with Preamp Disconnected
I disconnected my preamp yesterday completely from my system in order to install some constrained layer damping tape. The DAC was on and receiving music, but the signal chain between the power amp and the dac was completely broken due to the preamp being removed. BUT: I could hear very low volume, but very clear, music playing thru the speakers! And no, I was not intoxicated. How is this possible? Cool stuff. Thanks.