I haven't read the entire thread, but I've had the volume on my Dac3 maxed, and fed to either a t-amp or my headphone amp and couldn't hear any hiss with HD650s. I've only powered my e530s once or twice from it, but heard no noise either.
I usually have the volume on the headphone amp or t amp louder than I would normally listen and use the bel canto knob to adjust overall volume, out of laziness. Its usually in the 85-95% range.
There is a bit of hiss from the t amp by itself, that I can hear when everything else is turned off.