This past May, my best friend of 35 odd years asked me what I would recommend for a headphone amp. I told him "One that glows in the dark" - A vacuum tube based amp. I was excited by the idea of a new tube project. I researched dozens of designs and settled on an open source design (http://gilmore2.chem.northwestern.edu/projects/cavalli2_prj.php) mainly due to it's use of a medium high voltage supply (65v). Most other designs ran the tube on 24v or less. Yeah, that might still amplify the signal, but starving the plate of a tube will always result in unnecessary distortion.
Anyway, I found a dude in China who sold to me a couple of boards for the SOHA headphone amp. I sourced the parts and my friend and I each built one over a few weekends. Once I fired up the first build, I noticed a low rumble in the headphones during power up. With a volt meter, I confirmed that it was passing almost 4 volts DC to the headphones until the tube reaches it's operational temperature. This was not acceptable and could definitely damage a set of headphones. Thus my main mod was to add a dual pole "standby" switch to disengage the headphones during power up and power down. Other than that, my part selection was mostly as the SOHA project suggested. I used all Vishay Resistors, Nichicon Electrolytic Caps and 0.22uF Musicap coupling caps (the yellow ones in the photos).
Drilling the rectangular hole for the power entry connector:
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