I had same "accident" once, max. volume with 1 KHz playing in my head (I did to my cans "drop it like it's hot" instantly)...it was awful...I thought I'll not be hearing again, but until the morning I was fine, just some strange noises and tinnitus into my ears for the next couple of days.
Deckard and HPA-3U are sharing same schematic indeed, like many stated here and on Head-Fi too, just TTH vs. SMD components. Also, HPA-3B shares the same DNA, just a different way of interconnecting the output buffers (via 2 x dual opamps) and lack of USB transport and DAC.
My HPA-3B had some background noise and mains hum, but in a small margin...barely audible and with volume increased to the max. (inputs shorted for safety of my ears). However, noise was there, like Amir measured, so...this was the main reason why I choose to start
modding my HPA-3B.
Now it's much quiet and has no more audible mains hum, not even when using IEMs. Now it
measures pretty well, but it has no DAC inside, just the headamp.
P.S.: Mains hum is coming from the transformer, it's just too close to the electronic parts...a bigger case will help here, or shielding the transformer properly. The background noise is coming from the long audio traces from the RCA/XLR plugs to the ALPS potentiometer (full length/depth of the case) and then few inches back to the opamp inputs. Also, during this long audio path, lot of static and mains hum will get added, so cutting the traces and replacing them with shielded cables did helped.