Installed Nagra Reference amps, substituting the HD amps, everything worked properly, no unwanted noise thank goodness. The sound is fantastic, more present and more dynamic. What I found was that there are two very different presentations with output A vs Output B, Output B worked best in my system, tighter more focused. I found Output A warmer, perhaps too much bass presentation and the soundstage expanded more but not as focused and less pinpoint presentation with my current setup. The Reference DAC however has been a leap in dynamics, holography and presentation while maintaining the low sound floor that the HD had and that I like. Bravo Nagra