I guess I was lucky with mine from the first batch it ran nice and quiet even with sensitive phones. I should point out that I never tested the stock front tubes as fitted JAN-GE 5654W in the first week.
I use it mainly with HD800 Sdr modded headphones and feed it from the from the single ended side of the Chord Hugo TT DAC, the balanced side goes to a Sennheiser HDVA600, depending on the music and my mood I enjoy the differing presentation the tube and the solid state give me. I do tend to listen more to the tubes finding it compliments a wider range of music being slightly more relaxing yet still very precise.
Other headphones that sound good and benefit from the ZDT Jr I have found are HD 600 and 6XX HD 700 and surprisingly to me the Focal Clears from the low z output.
A bonus in this setup is it sits near to my powered desk speakers and the pass through works even when the amp is switched off